Black tank won't drain unless grey valve is open

Tank punch blank problem? Opening the grey valve pushes the blank back into the black tank?

while i doubt its a vent issue the easiest way to test is to fill the blank tank with water, pull the black tank valve, listen the flow rate, have someome hold the toilet pedal down, this will let air into the tank and the "boosh" will be heared.

When you have both grey/black drain valves open, Is the flow out of black/grey common drain header steady or does it kinda gurgle ??

If gurgles then black tank vent is not allowing air into black tank
Black tank drains using grey tank vent thru drain header....gulping air in

Turn water supply OFF
Try to drain JUST black tank ---open only its drain valve then have someone flush toilet and hold toilet open.......providing a VENT source
Does tank then drain?

Check vent line for blockage.......run garden hose DOWN vent line as far as possible then turn water on with black tank drain valve OPEN

Tanks have to vent...air out as liquids go in----air in as liquids go out

Could this be a clogged Black Tank Vent pipe that doesn't allow air into the Black Tank when dumping? Having the Gray Tank valve open would allow air to flow into the Gray Tank from its vent (assuming there is a separate one) or air gets sucked in via the P-traps. I wonder if would be useful to put a tissue over the Gray Tank drains eg. kitchen sink, bathroom sink and shower drain to see it it gets sucked in when the tanks are draining?
I admit these are WAG's on my part.

Can you post a pic of the pipes?

DrewE wrote:
When both valves are open, does the black tank drain or only the gray water tank? If it's only gray water, everything points to a clog in the black tank drain system--a fairly straightforward problem conceptually, but definitely not a lot of fun to fix.

If both tanks empty, you have something plumbed very strangely indeed or have a very peculiar problem with the tank vent system.

Both tanks drain when both valves are open.

Owned Trailer since new, I've been putting off trying to solve this issue. I've had over 20 issues fixed under warranty, and the quality of the build on this unit has been in question since I took ownership. It would not surprise me in the least if the valves weren't properly installed.

When both valves are open, does the black tank drain or only the gray water tank? If it's only gray water, everything points to a clog in the black tank drain system--a fairly straightforward problem conceptually, but definitely not a lot of fun to fix.

If both tanks empty, you have something plumbed very strangely indeed or have a very peculiar problem with the tank vent system.

Is this a new issue for this trailer, or has it been this way since you took ownership?

My first thought is that the plumbing was installed oddly. That the black tank has its own valve, but then the outflow of that tank also goes through the gray tank valve.

Jim