I recently had the same problem, where as poo water was leaking past the valve (3 month old trailer). I could not get the valve to close 100%, therefore allowing liquid to pass into the drain pipe. I figured something was lodged in the valve seat. After countless times off back flushing and filling and dumping the black tank the valve still leaked. I ended up removing the valve and found a cloth baby wipe lodged into the valve seat. After removing the wipe from the seat with a screwdriver, I reassembled the vale and has not leaked since.
Lesson learned: no more baby wipes in the toilet. This was a big job, as I had to cut a hole in the underbelly material to access the valve. Once all was back together, I scored a 2.5' x 2.5' piece of black coroplast material from a local sign shop and now have a nice access panel for future valve repairs.
Steve