Wednesday, February 1, 2023


Yikes! A holding tank pyramid!

By Chris Dougherty
Chris Dougherty is a certified RV technician. Here is a question he received from a reader while he was serving as’s technical editor.

Dear Chris,
We’ve been stationary for almost a month, so far. I’ve noticed what I think is a pyramid formed at the pipe base in the black tank. This has occurred twice in two weeks. I use one of those magic wands to clear it. We use probiotics in all tanks and Scott toilet paper. I use the flush king after every emptying. Any thoughts or suggestions? —Alan

Dear Alan,
This is a common problem, and you’re fortunate that you can actually see into your tank, as many RVers can’t. The problem is a result of too little water in the black tank. This can be an RVer’s error, or it can be a malfunction.

When it comes to sewage, water makes the world go ’round … and the sewage go down. Some people use no extra water in the holding tank because they’re afraid they’ll fill it too quickly, especially when dry camping. Others, especially if they’re on a seasonal site, leave the black tank valve open all the time, thinking everything will just drain. It doesn’t, which causes a buildup of sludge. Some folks won’t use chemicals, which can also do the same thing.

From the malfunction standpoint, the black tank system could have a leak. It could be as simple as a poorly sealing termination valve, or it could be more serious, like a cracked tank, or a failed pipe connection at the tank. If the tank has failed, there will be signs of leakage and odor. If your unit has an enclosed underbelly, there should be staining where the liquid has drained, and sometimes pooling which will weigh the underbelly enclosure down causing it to bow.

Finally, if the leak is in the valve, and this is the most common malfunction, it will be obvious when the cap is removed from the termination to dump the tank after use. If you are leaving your hose connected all the time, then there will be no sign of this, of course.

This is often caused by one of a couple of things: There is some stuff jammed in the o-ring part of the valve, the o-ring has failed, or the valve is in need of lubrication to make it close properly.

The best way to fix this is to service the valve, especially if it has been in use for a long time. To do this, disassemble the valve, clean the parts, replace the seals (a new kit is only about $8 on Amazon), reassemble and lubricate using Dow Corning 111 valve lubricant.

I hope this helps!



If you value what you learn from, would you please consider becoming a voluntary subscriber by pledging your support? Every contribution, no matter how modest, helps us serve you better. Thank youLearn more here.

Facebook Groups you might like
RVing with Dogs
RV Tech Tips
RV Advice
Towing Behind a Motorhome
RVing Over 70
. . . and the official Facebook page

Winterizing your RV this season? Amazon has a wide choice of RV antifreeze.


0 0 votes
Article Rating
Subscribe to comments
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
2 years ago

We’re on the opposite extreme from people that avoid #2 in their black tank.
When traveling with the RV, we never use any other toilet.

Traveling with our own toilet AND bed were the major reasons we bought a RV.

The only exception is avoiding using our toilet for #2 on the final leg of a trip back to the storage yard.

Debbie PJ
2 years ago

I have started washing dishes in Dawn in a bowl and then dump in toilet when done~ just to add extra water. We do try to use outhouses bathrooms for #2 as much as possible and always use camping paper.

2 years ago

We don’t have any of those problems with our Natureshead composting toilet. We also don’t flush water away.

Gregory Smith
2 years ago
Reply to  Robbie

I don’t know why RVers don’t use composting toilets more often; especially when boondocking. It doesn’t make any sense to me to use precious water to
Flush crap away. Might as well dump fresh water right onto the ground.

Bill K
2 years ago

I’ve learned that if the paper says it’s approved for septic tanks doesn’t mean it’s approved for a holding tank. Now it’s rv and marine paper only. And I agree that water is the key.

Don Lee
2 years ago

I don’t know how common this is, but when our black tank rises to the point where the vent pipe is blocked, we get a bubble when we flush. That’s our sign that its time to dump. I step on the flush pedal and fill the tank until I see water in the pipe just below the toilet. That along with the black tank flush gives enough force to rid the tank of most solids. We have been full timing for eight years and have never had a problem with this method.

2 years ago
Reply to  Don Lee

I’m not sure we cover the vent pipe but we do get the “burp” just before we get full. And, the SMELL usually comes with it…You KNOW the tank is full! Time to dump!

2 years ago

Yes to lots of water except when boondocking, of course. After traveling in Mexico a lot where you never flush toilet paper, camping or not, we don’t flush it in the rig any more. Bring on the Charmin.

2 years ago

Like Chris said in his opening statement, water is the key. We have owned motorhomes since 1996, and have never had a problem with our black tank. We also almost never used campground or public restrooms. We’ve got our own bathroom, why would we want to use a public one?
We have a connection on the black tank to hook a water hose to too rinse the tank, an I rinse it almost almost every time I dump it. If you don’t have a rinse connection on you black tank , put one on. It’s easy to do, I’ve installed them before. When I’m finished rinsing, I’ll close the valve and squirt about 4 or 5 gallons of water in the black tank. About 1 minute. Also, if I’m going to #2, I’ll hold the water only button in on the toilet and put some more water in the toilet. If we’re in a campground with full hookups, leave the sewer hose on, and the valve closed. I don’t let the tank get totally full, usually dump every 3 or 4 days.
Before you pull the dump handle, hook a hose to the connection on the black tank, (not the hose you use for your fresh water) and turn the water on. I’ll let it run for a few seconds, then pull the handle to dump the tank.
Before we leave the campground I’ll squirt a few gallons if water in the tank to let it slosh around while I’m driving.
I realize sometimes you have to use dump stations that aren’t the greatest, sometimes they don’t have another water connection to hook a hose to to rinse your tank. When you get someplace better, just rinse the tank then.


Bill Semion
2 years ago

if you are in a campground why not use the campground’s facilities??

2 years ago
Reply to  Bill Semion

That’s what we do and it helps get our steps in for the day. Only poop in the trailer when its a inconvenience to walk to the bathroom.

2 years ago

If possible, no #2 in the black tank. Water is your friend, not the enemy.

John McCullough
2 years ago

Maybe wrong toilet paper. We had trouble with Scott. And after getting things straightened out, we’re told to use Marine toilet paper. Have had no more problems.

Subscribe to our newsletter

Every Saturday and Sunday morning. Serving RVers for more than 20 years.