I have a rear kitchen model 5th wheel. There is a 2nd grey water drain/tank located at the back of the trailer. In my case, the handle is right behind the slide out.
It seems apparent that either you have a 2nd tank, or you have a plugged drain or pipe somewhere.
I've located the manual for my Mountaineer on the internet; maybe you can too, if you didn't get one. The model on Keystones is right by the door: mine is a 29RK.
Added: the handle fell off my drain once, leaving only a small threaded rod. Maybe the drain handle/rod is more subtle than what you are looking for.
