[Canonman]

Depending on the layout, there could be several feet of pipe between the black tank and the valve. We were told that could be the problem with our Cougar having some liquid behind the cap when we open it to attach the drain hose.
They are saying the pipe isn't completely drained.
Sounds like you have more than a trickle behind the cap.
Personally I like the idea of a terminal valve. I'm ordering one tomorrow to solve our problem. I hate surprises like the one you are describing.