Originally Posted by IronCobra
Valve inst leaking. Otherwise it would especially be leaking when the tanks are full. It must be liquid held in the plumbing after the valves. Probably just shakes loose while driving. It's appears to be my Kitchen sink grey water.
I had the same issue with mine. I'd empty it, flush it, empty it, flush again, empty it and wait 5-10 minutes and then put the cap on. Then had some nice surprises when open it at the next campground. Dealer put a scope up the line and found that part of the gasket was pinched when closed and allowing an extremely slow leak that would be waiting for me at the cap.
The movement of the camper would get whatever was left in the tank to drain and the valve would not hold it because of this tiny pin hole leak. They replaced my valve this winter and will be testing the new on Memorial Day weekend.