I have no direct advise to give, only my thoughts if this happened to me with our 2007 2350.
During winter preparation tank cleaning, I have never filled up our toilet tank so high that the waste water backed up right close to the toilet bowl valve. If I did and I discover a dripping, seepage under the motorhome, I would simply brush it off. In the real world, we would never allow the tank to get filled so high. Disturbing the system to better seal the waste pipe fitting on the top of the waste tank, would likely cause more trouble than the repair is worth.