This past summer I had a similar problem with the black tank leaking thru the valve. I was depending on the sewer cap to retain the nasties. I flushed the dickens out of the tank and took the drain plumbing apart. The whole problem was a hard deposit inside the valve that was preventing the gate from closing all the way. It was indicated by the handle shaft not going in all the way. After cleaning and reassembly, everything is now just fine. I found that there are many different versions of the thetford valve, depending on the year of manufacture and how it is attached to the pipes. All of them seem to have the same model number, so it can be very confusing to order one. It is very simple inside, so if the plastic is not obviously broken, I would recommend getting down and dirty. Just use LOTS OF WATER before opening the plumbing.
T.V.:'05 Dodge Ram 2500 4x4 Quad Cab Cummins