I have had that problem 2 times early on when my rig was new. Both times, the culprit was debris that had lodged into the drain valve seat (namely baby wipes), not allowing the valve to close completely. I had to cut hole in belly coroplast to access the valve, remove the valve and clean the debris out.
This was not a fun job and needless to say that baby wipes are now banned.
I'm not saying this is your problem. It could be that the rubber Oring valve seal could be twisted.
In either case, you will need to enter the darkside to diagnose or repair.
If you have a dealer do it, they will likley charge you and arm and a leg if it is debris related.