Some (not saying yours does) 5th wheels have three tanks....1 black, 1 grey (shower/bathroom sink) and 1 'galley' (kitchen sink). Each has their own dump valves. Sometimes the galley one is not with others and is back behind tires.
But if tank was full it would back up into kitchen sink.
The drain line from sink to tank could be cracked/broken.
The undercover is plastic. Look under kitchen sink and see where drain line runs.
Then with a mental picture of that location go outside and under rig.
You can cut a three sided window in the plastic cover (the non-cut side should be towards front-----2 parallel cuts lengthwise front to back with a rear cut across then pull flap down). This will give you a visual on the source of leak.
Got to find it before you can fix.......plus you need to get the water out of there and give it a chance to dry out (mold/mildew-----food debris if from kitchen sink)
The three sided flap can be taped back shut after repairs. Lots of flaps out there cause of needs to repair with enclosed/covered underbellies.
