Our black tank valve just recently starting "leaking" (all fluids would drain out)...had to use an additional blade valve temporarily to back fluids into the tank so it would not dry out. Before hitting the road again, we decided to replace ALL three valves and pull cables (no sense dropping the belly to replace just one). When we got the old valve out, we had the same problem. Apparently, over time some TP got caught on the valve and was repeatedly forced into the "seat" area for the valve, keeping it from closing. We have always used the flush and cleaned the tanks...religiously.
We use a biodegrading chemical that has lubricant in it and do everything possible to avoid this happening...and it STILL happened. Not sure there is any surefire way of avoiding this issue...some may never have it happen and somewill, unfortunately. However, it took five years for us to have this problem.
The one thing we did was make "fixing" this easier if it happens again. We used rubber connectors with metal covers and clamps at two strategic points where we cut the entire valve system out, so if we need to "clean" the valve out (next time we will know what's happening), we can drop the belly, disconnect the valve system, open up and clean the valve, and put everything back quickly...no fuss, and a relatively quick job....hope this helps.