Above and Below: Understanding the RV Waste System

Graphic diagram of RV waste system including pipes and tanks.
Wastewater System: The typical RV sanitation system is made up of a black-water holding tank for solid waste and at least one gray-water holding tank for liquid waste. Illustration: Rob Roy

With a little practice and forethought, RV waste systems will be easy to use, and the experience can be as pleasant as possible.

Nothing portrays affection more than being the person responsible for dumping and maintaining the RV holding tanks. And yet, how many owners truly understand what makes the system tick and how to keep it performing at its best?

First of all, it really isn’t rocket science. It’s just science. We use water throughout our day to hydrate, to wash and to eliminate. In fixed structures, many people forget about the process after the contents are flushed away, but with an RV, this is only the first stage of the sewage-treatment process. We store the waste, treat it and systematically send it into an appropriate receiving system, commonly known as a dump station. Done correctly, the system remains relatively clean and operational with little fuss. Done wrong, solids can build up in the tanks, which can lead to trouble.

Above the Floor

Photo showing foam gasket on bottom of RV toilet
Unlike residential toilets, RV counterparts use a foam gasket instead of a wax seal. It is recommended to replace the gasket each time the toilet is removed.
Photo showing control handles for RV holding tanks
RVs with hidden tank valves will operate using remote, cable-controlled handles. The handle attaches to a frame that connects to a shielded bicycle-brake-type cable.
Photo showing RV articulating hose connection.
Some RV builders will mount the tanks in a heated area, and the discharge comes directly off the tanks. The compartment hatch has to be opened to connect the sewer hose to an articulating hose connection.
Photo of RV black tank pipe and valve
Most trailers will have a 3-inch pipe that comes straight out of the black tank, which makes attaching a clear external flush fitting (not shown) a practical option. The gray 1½-inch pipe goes into the valve at a right angle. For ease of identification, handles on this RV are color-coordinated (black for waste and gray for wastewater). Another variation has the gray line coming in at a 45-degree angle.
Photo showing RV sewer macerator connection
Macerators can be installed or twisted on to the sewer outlet and allow waste to be ground up and pumped to a disposal site.

When looking at a diagram of a waste plumbing system in the average trailer, it’s pretty simple. The system is usually divided into two waste systems: the black system, which is solid waste, and the gray system, which is liquid waste from sinks and the shower.

The black waste system is usually made up of 3-inch black PVC pipe that runs from the toilet down to the holding tank. In most travel trailers, there’s almost no pipe; the toilet flange is glued to a threaded fitting that attaches directly to the tank. Fifth-wheels will have a pipe from an upper bathroom routed down to the tank. In all cases, the tank is vented through the roof.

The gray waste system has 1½-inch black PVC pipe that is routed from each water fixture, sink, shower and clothes washer to one or more gray holding tanks. Each will have a P-trap or a HepVo device to prevent gas flowback. A HepVo is a waterless valve that will prevent gas and tank overflow liquid from backing up into the shower or sink. The most obvious benefit is that, unlike a P-trap, water is not required to maintain the seal, which means the chance of odor coming back up into the RV when water splashes out of the P-trap is reduced to zero. The HepVo also requires much less physical space to install, and winterizing is not needed.

The gray waste system also needs ventilation to work correctly, to vent the tank and allow water to properly drain. Modern RVs reduce the number of roof fixtures by using small vents that are usually hidden in walls or cabinetry. Anti-siphon trap vent devices (ASTVD) are attached to a vertical pipe near the appliance and have a neoprene or rubber flap inside that opens when there is a vacuum. Higher-end models are spring-loaded. ASTVDs are maintenance-free; however, if there is an odor problem in the RV, especially from gray water, maintenance or replacement may be required. The ASTVD can be twisted off, via threads, for replacement. If the rubber has become misformed, the unit should be replaced.

To save weight, most plumbing components in an RV are plastic, but they rarely fail.

There are many models and styles of freshwater toilets on the market for RVs. They range from plastic economy models to full china-bowl electric-macerator flush models. While the latter usually depends on 1-inch rubber hose that’s routed to the black-water tank, the others connect to the aforementioned toilet flange on the floor. The toilets will have a two-position flush valve; the first fills the bowl with water, and the second flushes the bowl, which is done either with a hand lever or a foot pedal. When flushing, a sliding blade or ball valve opens to empty the contents straight down the pipe.

The toilet flange is similar to the one in a stationary home, but the seal is made of foam rubber rather than wax. If the toilet is removed for any reason, the best practice is to replace the foam-rubber seal. However, if it passes a careful inspection and is reinstalled just as it came out, it should be OK.

Below the Floor

Graphoic of RV holding tanksIn a house, the soil stacks run into the cellar or below the foundation and then out to the sewer or septic system. Once you open a drain or flush the toilet, there’s nothing more to think about. In an RV, wastewater is collected in holding tanks by type: “gray” for the sink and shower water and “black” for sewage. The tanks are piped to termination valves and a fitting to which the sewer hose is connected.

Tanks are usually made of polyethylene or ABS plastic, and pipe fittings are either glued or spin welded in place, depending on the type of tank. The tanks are vented through the roof via a 1½-in pipe.

Some RVs have more than two holding tanks. This is not just for added holding capacity but to accommodate certain floorplans and for plumbing slope. For a waste pipe to drain correctly, it has to maintain a slope of at least 1⁄8-inch per foot, as required by the NFPA 1192 building code. Depending on the length and style of the RV, there may not be enough vertical drop to maintain that slope.

RV Sanitation Resources

Holding-Tank Sensors
Garnet Technologies (SeeLevel), Horst Miracle Probe

Holding-Tank Treatments
Camco (TST), Century Chemical (Travel Jon), Dometic, Eco-Save, Star brite (Instant Fresh), Thetford (Aqua-Kem, Campa-Chem, Eco-Smart), Valterra (Odorlos, Potty Toddy, Pure Power), Walex (Bio-Active, Bio-Pak, Commando, Elemonate, Porta-Pak), Worldwide Monochem (Dyna-Bact, T-5)

Clean Dump, Thetford (Sani-Con Turbo), Valterra (Sewer Solution), Xylem (Flojet)

Sewer Hoses
Camco (RhinoFlex), Drain Master (Waste Master), Lippert (Waste Master), Prest-o-Fit (Blueline, DuraForm), Thetford (Titan), Valterra (Dominator, EZ Flush, Quick Drain, Viper)

Termination Valves and Parts
Barker Manufacturing, Camco, Drain Master, LaSalle Bristol, Valterra

Dometic, Thetford

Waste-System Parts
Oatey (ASTVD/In-Line Vent, PVC/ABS Cement)

For example, a travel trailer might have only 3 to 4 inches from the top of the floor to the top of the tanks. Since the tanks sit right up against the bottom of the floor, the drain plumbing must be run above the floor, or, in the case of the toilet, drain directly into the tank. If the waste facilities in the RV are far apart, the solution is often to install multiple tanks. When configured in this manner, the waste systems are entirely separate from each other. Some motorhome manufacturers use macerator pumps to move waste from a passenger’s-side discharge tank to a driver’s-side sewage-dump connection, again, because of slope, and often a motorhome chassis is in the way of running directly to the discharge.

Some RVs are equipped with a black-tank flush; aftermarket systems can also be retrofitted, which is a great option, since it helps rinse solids from the inside of the tank. The flushing system usually runs from a garden-hose connection on the side of the RV directly to the flush nozzle via hose/tubing and an inline vacuum breaker to prevent any chance of backflow. Line pressure works fine for a flushing system, and best practice is to use a hose for rinsing duties that will not be used for potable water. Varying lengths of gray hose are available from RV-supply stores, or you can make one from an old hose — making sure to mark it clearly for this purpose.

The diameter of the black-tank discharge pipe is 3 inches; the gray tank is 1½ inches in most cases. If a gray tank has its own sewer connection, it may have a 3-inch discharge pipe. The sewer-hose connection, however, remains the same. For those using a “gray-water pit” (where it is allowed), a replacement sewer cap with a garden-hose fitting is available. It is recommended to make a ¾- or 1-inch hose for this purpose using parts that can be found at a home-improvement center.

Many RVs are designed and built for “all-season” use. This can be a bit misleading because, although some RVs can usually do well in mild winter and snow conditions, most cannot handle severe weather. One of the ways manufacturers help to accomplish this is to enclose the dump valves and piping in a heated and enclosed underbelly, and add electric heating pads to tanks and plumbing. Often the dump valves will be remotely controlled, and some will be electric. The basic design and operation of the valves remain the same.

Dump valves in RVs use a sliding blade that minimizes the opportunity for clogging while being easy to disassemble and service. When the system is built, a flange is glued to the waste pipe on each side of the valve. Gaskets sit in a groove in these flanges, and the valve is inserted between the flanges and secured with four bolts. Should the valve fail to close, or if it leaks, the fix is pretty straightforward. Dropping the underbelly may be required.

Termination valve sets, where both valves are attached to a common, sometimes articulating sewer-hose connection, are relatively common. In trailers with this type of connection, the valves are usually exposed to the outside air, and thus can freeze, while some larger motorhomes will have this type of connection in a heated compartment.

Making the System Work Better

The black tank can be the most problematic. These tanks can collect solids and become a source of odor. Water is your friend here. It, along with an effective toilet treatment, is what allows the tank, and nature, to do their work. Too little water, and solids pile up on the bottom of the tank and can accumulate to the point where they prevent complete tank flushing, which is nasty and will produce an equally nasty repair bill.

Always start with a flushed tank and add a few gallons of water along with a preferred toilet treatment. Do not be tempted to use homemade concoctions in the tank. There’s plenty of bad advice on the internet, and mixing the wrong chemicals can be damaging to some components in the waste system, such as valve seals. Using anything but chemicals designed for RV holding tanks can create hazardous and even deadly gases, which can lead to system damage — and affect septic systems tied to dump stations. Again, add plenty of water to the toilet each time solids are introduced, and make sure there is plenty of water in the black tank before dumping.

See Related Stories:

Small Favors

Dirty Little Secrets: RV Sanitation

Macerators, like those from Thetford, are becoming more popular in RVs. Portable models can be attached directly to the sewer-hose connection and are designed to pump the effluent through a 1-inch hose routed to the dump station. It will pump uphill to a certain degree and can offer the flexibility of dumping tanks into a residential cleanout. Whichever hose system you use, always drain and flush the black tank first, then the gray. Do not open both valves at the same time.

Chris-Dougherty-headshotChris Dougherty is technical editor of Trailer Life and MotorHome. Chris is an RVDA/RVIA certified technician and a lifelong RVer, including 10 years living full time in an RV. He and his wife make their home in Massachusetts and hit the road in their heavy-duty truck towing their travel trailer every chance they get.


  1. The water connection to my toilet has started dripping when I flush. It leaks from the top fitting, and is firmly tightened. What should I check to remedy this problem.

    • Chris Dougherty replies: It is likely the check valve has a crack. This is especially common when antifreeze is not used to winterize the unit, but it can also happen if the plastic check valve is stressed.

  2. One of the nipples on the waste-water outlet pipe is missing, the lower one. So when I open the dump valves, the connection between the sewer hose and outlet pipe drips. How difficult is it to replace the end piece, and do I have to cut the old one off and reglue the new one?

    • Chris Dougherty replies: Scroll down to see James Prekoski’s comment in response to your question, Peter. I agree with James. A cut-off tool or Dremel tool can help make a smooth surface. The glue is available at Home Depot or other home centers.

      • I’m building a 12-foot RV, and space is limited. Is it okay to have one waste tank for everything? My plan includes a 50-gallon black/gray tank and a 40-gallon freshwater tank.

        • That’s a question for our technical experts at RV Clinic. You can email your question to RV Clinic at [email protected]. Selected letters will be answered in the magazine and published on the website. Please include your full name and hometown in your email.

  3. I know the article is about the wastewater system. Is there a similar article about the water-supply system? When connected to city water, sometimes my freshwater tank overflows. I assume the demand pump has a check valve in it? It does not overflow all the time, just sometimes. Is there something else involved?

    • Chris Dougherty replies: We’ll add an article on the water-supply system to our to-do list, Michael. As to your question, I assume you have a fifth-wheel or motorhome with a tank fill valve in the utility bay. It could be that the valve is positional and finicky, or is really leaking through the valve? I would try having the unit connected to city water and play with the water tank fill valve. You should be able to hear the water flow past the valve as it goes into tank-fill setting. Try to tell if water still flows with it in the normal setting. If so, the valve will need to be replaced.

    • Chris Dougherty replies: Dan, most but not all of them do. Some high-end fifth-wheels and motorhomes have a tank fill connected to the plumbing system near the city-water connection. When the valve is switched, it fills the tank via a PEX pipe. Some RVs have both a gravity and a city fill.

  4. Don’t have to cut pipe. There is a repair piece available to glue to the existing pipe end. Must file off the remaining nipples to make a smooth end to glue on the new piece. The one I used is Valterra T1029-2 Termination Adapter – 3″ Bayonet x 3″ Hub. Bought from Amazon.

    • Chris Dougherty replies: I agree! [Note that James Prekoski’s comments are in response to Peter Bradley’s second question above.]

  5. Chris (or TrailerLife): That s a beautiful diagram! Is it possible to get a copy to it without the title in it? Great article!

  6. Am I the only one that has pictures from the Above and Below article that cover the middle of the page so quite a bit of text?

  7. No, you’re not. That happens to me as well. Haven’t discovered a ‘fix’ for it. It’s quite irritating to ‘fill in the blanks.’

  8. This is a helpful and informative article. I also have the pictures covering text. But we can get the drift of the article. Thanx for this.

  9. Good article except not stressing the fact that one should NEVER LEAVE THE BLACK-TANK VALVE OPEN while in camp. I see many newbies do this and don’t know how to say anything to them. I just hope I never purchase a used rig from someone who does this.

  10. I just purchased a 2019 Forest River 26-foot travel trailer. First black-tank flush, and we got a clog. When viewed through the clear 90 at the end, I saw a piece of plumbing. After I cleared the clog, I found a 1.5-inch threaded male adapter. I’m concerned if something broke or someone at the factory just threw an old connector in a black tank. Is there 1.5-inch plumbing connected to the black tank?

    • Chris Dougherty replies: Sorry to hear that. If you’re not getting a lot of bad odors or worse inside the RV, then it probably fell in there at the factory. There is a 1.5-inch fitting in the top of the tank for venting the tank to the roof. While these are not normally a threaded fitting in my experience, there’s nothing to say that it couldn’t be. If the 1.5-inch vent pipe broke loose, you would have really bad odors and overflow into the underbelly of the trailer.

  11. Isn’t this diagram incorrect? How are you venting the air trapped in the gray-water tank? Shouldn’t there be another vent to the roof, which brings me to my question: can I use a single vent for both black and gray?

    • Sorry for the confusion, Ricardo. A previous version of the diagram was inadvertently posted on the website. The correct diagram is now online.

      From Technical Editor Chris Dougherty: Every holding tank needs to have some manner of ventilation. The gray- and black-water holding tanks need ventilation to the roof, but I see nothing in NFPA 1192 indicating that both tanks can’t share a common vent. However, sizing has to be taken into consideration. That said, best practice would be to have individual vent pipes, a minimum of 1¼-inch, extending from the highest point in the tank to the roof. Also, as a side note, the code specifically states that no connection between the body waste and liquid waste tanks shall be made above the fullway termination valve.

  12. I removed the old leaky black-water valve under my fifth-wheel and am having a difficult time getting the seals to stay in place to put new valve in. The space between the black-tank pipe and drain pipe is so tight, every time I get the valve in the seals have moved. There is no way to see it, as it is under floor and covered by underpinning working through a hole cut to have access to the valve. I used petroleum jelly first, and then tried plumber’s grease. Only way to know they have moved is to back-flush and watch for it leaking. Appreciate any advice. Prefer not to cut the drain system.

  13. I am having a problem with the shower not draining in our 2004 Alfa Sun fifth-wheel. The bathroom sink drains fine, and no problems in the kitchen. I have removed the trap fitted to the bottom of the shower drain and made sure it was not plugged there. Please help with where or what I need to do next. We are full-timers, and this is our first problem since we’ve been living in the fifth-wheel for two years. We leave gray and galley valves open and black closed till we empty black, and then we close the other two valves and open and flush black. Please help.

    • This is a question for RV Clinic, Trailer Life’s forum for RV-related technical questions. Send an email with your question to [email protected], and include your full name and hometown. Selected letters will be answered in the monthly column, but time does not permit individual replies.

  14. My sewer system is not draining. The toilet valves open. Must be plugged inside? Or do I need to turn on fill tank? I do not understand what the lever fill tank is for? It is next to the handles to pull to drain. When I turn it on, the pump runs.

    • This is a question for RV Clinic, Trailer Life’s forum for RV-related technical questions. Send an email with your question to [email protected], and include your full name and hometown. Selected letters are answered in the monthly column, but time does not permit individual replies.

  15. I’m glad you explained why we should avoid adding homemade concoctions to a freshly flushed RV tank. My husband and I have an old RV that we want to have flushed-out before the weather warms up so we can start using it again. I’m glad I read your article and learned some good maintenance tips to keep in mind after we have the RV repaired!

  16. First of all, thank you for this great walkthrough and for providing a space to share and learn ideas.

    Second, I bought my first RV, a fifth-wheeler, a few days ago. The plan is to keep it stationed on a property of 5 acres, and right now my idea is to “marry” the gray-water and black-water tanks, and replace the toilet with a urine-diverting, composting toilet, the idea being that I could run all waste water to a buried tank in the ground, and the solids buried to be decomposed and reused as fertilizer, but…

    My question is, if I had the septic tank set up and connected to the RV, would I still need to do any flushing? Or would everything simply gravity-flow into the septic tank?

    If you have any other suggestions for diverting urine and composting solids (or avoiding use of the black-tank) please let me know. Thank you.

    • Mike: That’s a question for RV Clinic. Email your question to [email protected] and include your full name and hometown. Selected letters will be answered in the magazine and published on the Trailer Life website.

  17. Do all holding-tank vents have screens in them to prevent hornets, mud daubers and wasps from going down into the vent pipes?

    • Jerry: That’s a question for RV Clinic. Email your question to [email protected] and include your full name and hometown. Selected letters will be answered in the magazine and published on the Trailer Life website.

  18. DIY schooler build. Composting toilet. Hepa drain (check) valves. Boat shower boxes collecting drainage and pumping to holding tanks. One tank for gray. One for black (liquid only). Question is: How small of a vent tube can I run to the roof vent for my black and gray tank? Thank you for your time.

    • Ryan: That’s a question for RV Clinic. Email your question to [email protected] and include your full name and hometown. Selected letters will be answered in the magazine and published on the Trailer Life website.

  19. Is it safe and acceptable to spray fresh water via my garden hose down through the plumbing vent stack, which is located on the rooftop of my motor coach? I was hoping that this would help loosen up solids that may be accumulating inside the black holding tank. Thanks.

    • Technical Editor Chris Dougherty responds: In theory, yes. However, there are two concerns. First, it’s possible the vent is tee’d off somewhere and not a straight run to the tank. Second, the 1½-inch pipe gives only a small access through the top of the tank versus the toilet pipe/fitting. The only time I would consider flushing water down the pipe is to clear the pipe of insect nests or other debris, not to clean the tank.

  20. You show the black and gray tanks sharing the same vent stack. The problem is, if the black tank is overfilled, in your drawing it will overflow into the gray-water tank before it starts to show up in the toilet bowl. You need to redraw your diagram and show the gray tank vent connecting higher on the vent stack. That way when the black tank is overfilled it will show up in the toilet first before it overflows into the gray tank. Given your drawing as it is now, overfilling the black tank would completely overfill the gray tank with sewage before it would show up in the toilet and might also first come out in the lowest drain, the tub drain.

    • Chris Dougherty replies: Jim, magazine art can be a challenge because of space. Most RVs today have separate vents for each tank. We also didn’t show second and third gray tanks or the myriad of other combinations that are out there. I have seen where the vents are connected, but its rare. Also, there are units where gray and black go to one tank, or some gray goes to the black tank Thanks for studying it so closely!

  21. I have a 23-foot Pioneer RV and am getting rainwater filling my black-water tank every time we get a heavy downpour, then it backs up and out of the toilet.

    • Todd: That’s a question for our technical experts at RV Clinic. You can email your question to RV Clinic at [email protected]. Selected letters will be answered in the magazine and published on the website. Please include your full name and hometown in your email.

  22. Good article. I’m in the process of restoring an old 1979 trailer. The trailer doesn’t have any underfloor holding tanks. The toilet is mounted on a “step” under which is a small above-floor, black-water tank, about 5 gallons.
    I’m thinking of installing a regular, higher capacity underfloor black-water tank, but due to the small size and configuration of the trailer, the toilet sits right inline above the trailer frame. To reach the tank, I would need to put a 90-degree elbow right under the toilet to clear the frame, then another 90-degree elbow down to the tank. All RV toilets I’ve seen are flushing straight down in the tank, so would those two elbows cause any flushing issues, clogging, etc, or is this acceptable?

    • Carl: That’s a question for our technical experts at RV Clinic. You can email your question to RV Clinic at [email protected]. Selected letters will be answered in the magazine and published on the website. Please include your full name and hometown in your email.

    • Kriss: That’s a question for our technical experts at RV Clinic. You can email your question to RV Clinic at [email protected]. Selected letters will be answered in the magazine and published on the website. Please include your full name and hometown in your email.


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