I occasionally experience a blockage of my black tank when draining. We use "normal" house toilet paper for a myriad of reasons. Yes, I know it's not recommended. But I found a quick and easy way to clear a blocked line. I use the gray water tank, it should be full or nearly so to accomplish this.I purchased an accessory gate valve which connects to the plumbing drain line and then connect my dump pipe to that. This allows me shut off the dump flow while leaving the gray/black valves open. Don't panic yet, I'm not contaminating grey with black. If I see I've got a black blockage I close all valves, wait until the flow stops and then close the accessory valve. Then I open the grey water tank until the pipes are full, usually gurgling followed by silence. Then I open the black water valve. The pressure of the gray water is greater than the black and the gray water backflows into the black tank. I wait a about 30 seconds, then close the gray water valve and then open the accessory valve.This works great for me, never failed to clear a black blockage in seconds.

Perhaps you need to use more water when flushing.

Better yet, try one of these:Tank Rinser

I agree with 2oldman, it sounds like a lack of water problem rather than the TP being used. We've used Charmin Ultra Soft for many years in multiple RV's with no problems. Pretty much any TP labeled "Septic tank safe" or similar should work fine with enough flush water. That said, we also have a third twist-on valve in place that we use to either back flush the black tank just for more thorough cleaning or to allow the tanks to equalize when we need additional capacity when no sewer hook up is available. It's usually the grey tank that needs the extra capacity, so the flow is normally from the grey to black in that situation. I don't have any concerns with the two tanks "cross contaminating" though, since it's all sewage that's going to end up in the same place.

2oldman wrote:Perhaps you need to use more water when flushing.Bingo. And once again, H2O to the rescue.:W

In addition to obviously needing to use more water when using, I mounted one of the flush valves on the side of our black tack in years past when we have a 5th wheel. Use clear silicone sealant and "snug" the mounting screws. Do not over tighten!! Ours actually sprayed all over the inside of the tank and did a pretty good job. We used it about every 2-3rd time when dumping depending on how many other folks were waiting. I really don't see how the ones mounted on the clear discharge valves can get up inside the tank very well. Another possible option is a wand type device with an on/off valve that is attached to a water hose and then brought into the bathroom via through a bath window or through the front door and stuck down the toilet before turning on. Sounds kinda gross to me.https://www.rvsupplyparts.com/catalog/rv-holding-tank-flush-kit/valterra-no-fuss-flush-holding-tank-cleaner-a70-7164.html

Easy Backflushing to Clear a Clogged Black Tank

12thgenusa
Explorer
Jun 17, 2017
You guys talking about not using enough water are barking up the wrong tree. It's a plumbing problem not a water problem.

The OP's trailer is probably plumbed in a similar fashion to mine. The black and gray tanks are side by side with the drains on the sides of the tanks facing each other and are are coupled with a tee. From tee there are two more 90 degree turns before the exit point. So waste has to essentially make four 90 degree turns within a very short span. Flow rate and velocity are greatly reduced as a result.

I've been doing as the OP suggested for years and it works great.

Those of you that have a tank with a straight shot out the end won't have this problem.
pauldub
Explorer
Jun 17, 2017
Are you camping with hookups and leaving the black valve open all the time? That's the most common way to cause a blockage problem.
Ron3rd
Explorer III
Jun 16, 2017
In an ideal world we would never get a clog, but it does happen. Your method, the Flush King, or the Camco device all work for back flushing. BTW the vast majority on this forum do not use the "RV" toilet paper either.
Old-Biscuit
Explorer III
Jun 16, 2017
I have permanently added an 'extra' valve on trailer dump line for many years/several trailers.

I use it to back flush black with grey
I use it to combine grey/galley
I use it to combine all three...black/grey/galley for extended camping

Waste tank is a waste tank..grey is just as nasty as black.

The type of TP is not your blockage issue.

Liquid to solids ratio is.......need more liquid and dump when tanks are near full (fuller the better....high volume/weight increases the 'whoosh' factor)
JC2
Explorer
Jun 16, 2017
In addition to obviously needing to use more water when using, I mounted one of the flush valves on the side of our black tack in years past when we have a 5th wheel. Use clear silicone sealant and "snug" the mounting screws. Do not over tighten!! Ours actually sprayed all over the inside of the tank and did a pretty good job. We used it about every 2-3rd time when dumping depending on how many other folks were waiting. I really don't see how the ones mounted on the clear discharge valves can get up inside the tank very well. Another possible option is a wand type device with an on/off valve that is attached to a water hose and then brought into the bathroom via through a bath window or through the front door and stuck down the toilet before turning on. Sounds kinda gross to me.
https://www.rvsupplyparts.com/catalog/rv-holding-tank-flush-kit/valterra-no-fuss-flush-holding-tank-cleaner-a70-7164.html
JC2
Explorer
Jun 16, 2017
2oldman wrote:
Perhaps you need to use more water when flushing.

Bingo. And once again, H2O to the rescue.:W
Dutch_12078
Explorer II
Jun 16, 2017
I agree with 2oldman, it sounds like a lack of water problem rather than the TP being used. We've used Charmin Ultra Soft for many years in multiple RV's with no problems. Pretty much any TP labeled "Septic tank safe" or similar should work fine with enough flush water. That said, we also have a third twist-on valve in place that we use to either back flush the black tank just for more thorough cleaning or to allow the tanks to equalize when we need additional capacity when no sewer hook up is available. It's usually the grey tank that needs the extra capacity, so the flow is normally from the grey to black in that situation. I don't have any concerns with the two tanks "cross contaminating" though, since it's all sewage that's going to end up in the same place.
STBRetired
Explorer
Jun 16, 2017
Better yet, try one of these:

Tank Rinser
2oldman
Explorer II
Jun 16, 2017
Perhaps you need to use more water when flushing.

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Easy Backflushing to Clear a Clogged Black Tank

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