Waste water

Residue that is still on inside of drain lines AFTER the dump valves......or one of the waste valves leak thru IF you are really getting a 'few gallons'

Usually the black tank valve due to stuck TP in drain valve blade grove which doesn't let blade fully seat

A few drops would be common, a few gallons isn't. When you flush leave the value closed for a minute to get more water in the black tank to clean the valve when you open it.

I think the black and/or grey tank valves are leaking.

^^ yep. Somethin's leaking. Is it gray or black water?

The only way I can see sewer backing up at the cap, is if the valves aren't closing all the way AND there is fluid being put into the tanks. Has any water been used and dropped into either tank while the cap is on and in travel? If it were me, I would try and not put anything into the tanks while on the road and see if the couple of gallons are still there at the next setup. Everything should be dry - except for a couple drips.

Could be the bottom of the tanks are sagging and driving down the road sloshes the fluid enough that it goes down the pipe. I know for a fact this has always happened with ours and I suspect it's very common.

Moved from Forum Technical Support

I added a twist on valve at the end of the dump pipe. I never had a large volume of rogue liquid, but enough to be annoying and this addressed that. It also allows you to blow some grey water up into the black tank for a little extra flush action if you're so inclined.

You should catch the waste water in a bucket, when arriving at the next campground, and first remove cap. It may seem like a couple of gallons when it comes spilling out, but may not be as much as you think.

My FW will have a quart or two of residual water stay in the drain lines, and spill out when cap is first removed after traveling. Once this is out, I can put cap back on, let all 3 tanks refill for a while, trailer parked a couple days, take cap off, nothing more to come out.

If you really have a couple gallons spill out after travel, I would suspect a leak. You can add a twist on valve at the hose connection to avoid this, until problem is solved.

Jerry

IdaD wrote:
I added a twist on valve at the end of the dump pipe. I never had a large volume of rogue liquid, but enough to be annoying and this addressed that. It also allows you to blow some grey water up into the black tank for a little extra flush action if you're so inclined.

X2!
I have a grey tank valve that leaks, and is nearly impossible to to remove as the Back and grey tank outlets face each other, and tie into a Y fitting I would need to move one of the tanks about an inch or two to get a new valve in.