There is a the rubber seal around the 'ball' that is not working correctly to hold the water there. See if the seal has been compromised in some way or is there any debris on it to where the ball is not closing correctly. The toilet can be taken out very easily and flipped over and the seal removed and cleaned and reinstall. If the seal is messed up a new seal can be ordered for the toilet unit.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler