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(photo courtesy of U.S. Forest Service)
(photo courtesy of U.S. Forest Service)

Mark My Words: Dealing with holding tank blockage

This question and answer is provided by Escapees Club.

Q. I’ve had my RV parked while being used for about three weeks and when I went to empty the black tank, the tank only drained for about one to two minutes before stopping. The tank level indicator light had shown the tank was full before emptying, so I expected that it would take five minutes or longer to completely empty.

I suspect there may be a blockage in the drain pipe. Do you think this is the case or could it be something else? If the pipe is blocked, do you have any possible solutions to correct the problem? I have thought of driving around to loosen up any material in the tank/pipe, but I was hoping to come up with a solution without having to move the RV.

For the future, do you know of any good tank additives to keep all solids, including paper, dissolved for easy flushing?

A. First, be sure you have a problem. It should only take a minute or two to empty your tank. It’s draining through a three-inch pipe, so it should go pretty fast. If it’s a straight drop from your toilet to the tank, shine a flashlight down there while holding the flush valve open with your foot on the pedal. The tank may actually be empty. If not, read on.

Sometimes, using the wrong toilet paper, or too much of the right toilet paper, can cause a clog. In most cases, you can shift it with a flush fitting that attaches to the sewer-hose connection and back-flushes the pipe. The picture is of the RV Hydroflush from Valterra. Most RV parts sources carry them.

In the meantime, here are some general tips to help prevent clogs:

First, use enough water each time you flush to ensure that the waste is transported to the tank. Use the “add water” pedal, or lever, to partially fill the bowl before depositing solids. The extra water will help carry everything along quickly to the tank.

Second, use a toilet paper that actually dissolves. Either buy RV toilet paper or pick a brand that breaks down fully. If in doubt, do this simple test. Take a sheet or two of your toilet paper, put it in a jar half-full of water, tighten the lid and give it a shake. Safe toilet paper will dissolve readily. If it doesn’t, it shouldn’t be used in your RV! It may also help to use toilet paper sparingly.

Finally, use a bacterial or enzyme-based tank treatment like Odorlos, Enviro-chem or Pure Power. They are good at liquefying waste.

Always let the black water build up until the tank is at least one-third full before you dump it. Waiting to dump the tank keeps solids in suspension, and the quick rush of fluid out of the tank when it is dumped helps carry most of the solids out. Whenever it’s convenient, try to dump the black tank after traveling. The motion of the rig on the road will mix up the contents nicely and help break down the solids.

About Mark Nemeth

In 1997, Mark quit his aerospace engineering job and became a full-time RVer. For almost five years he traveled the country while maintaining his website, Mark’s Fulltime RV Adventure ( posting monthly travel logs. Mark now travels part-time and works as the RV Safety Education Director for Escapees RV Club providing technical training at Escapees Boot Camp seminars, as well as assisting in many other areas of the club. He is also technical advisor for Escapees magazine. Learn more about Escapees RV Club at

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