Black Holding Tank slow to drain...

2gypsies wrote:
If the liquid rushes out at first and then slows, could it be that it's just empty?

We have a 5" clear straight extension on the sewer outlet, at the tank. When the hose slows, the clear extension is full. I can see waste moving through the extension slowly. The sewer hose is full of waste. I can lift it and feel that it is full the entire length of the hose.

Old-Biscuit wrote:
Unless a bathroom sink or shower has been plumbed to black tank no 'AAV' on black tank. Typically grey only.

Tanks that start out draining OK then slow way down are due to tank vent being blocked OR the cut-out disc was left in tank and gets sucked down to drain out let blocking it.

Thanks Old-Biscuit. I wasn't sure about the AAV, we have one in the bathroom, so I wasn't sure if it was for the toilet or the sink. I haven't seen another in the camper. Thanks for verifying.

My original thought was the vent, but I wasn't sure. I did inspect the vent exhaust and it looked fine. Thanks for the suggestion of the garden hose. I'll give that a try in the morning.


Our site is a seasonal site, no one else has used it since 2014. The previous site occupants occupied it for 5 years. They were an older couple (in the 80s) and only stayed a few months of the 5 month season. If they had clogged it, I think we would have had a problem last year.

Since our camper is over 7 years old, I think we would have found the "cut disk" sooner. The waste flows just fine out of the tank until the hose fills up. There is no leaking at the ground connection. The pipe is a few inches above the ground and it's a tight seal.

Thanks all.

-Michael