Liquid waste knife valve leaking (not the black tank)

A bit of rubble in the grove is my guess, too. Could be 'bout anything. My black tank had a similar problem, that turned out to be the plastic cutout from when they drilled the hole in the top of the tank to install the toilet. It was "found" near the end of a thorough cleaning/flushing, so when I had to reach up the pipe and pull it out, it wasn't entirely nasty.

Rinse the tank well, that might dislodge it. Fill the tank FULL of clean water and do a complete dump. If that doesn't clear it, you, too, might be a candidate for reaching waaaaaaayyy up in there and running a finger around the groove . . . .

If you find the answer let me know , have same camper with same problem. Design flaw ?

Probably have something in blade valve grove.
Cable operate valves will not close at tight when stuff builds up in grove due to flex in cable.

Sometimes you can clean out grove by heavy flushing or by back flushing while partially open/closing valve.

If seals are cause of leak.......usually end up having to replace.

They make a cap with a hose fitting on it to back flush the valve in case something that went down the drain is trapped at the valve, preventing it from closing all the way.