The RV commode is supposed to trap some water in the bowel. The trapped water prevents noxious odors from entering the RV which is a way good thing. If the water leaks out and the bottom of the bowl is dry, you can "renew" the flap by cleaning it. Turn off the water. Apply rubber gloves, then push the rubber seal down about one quarter inch with your fingers. Use the blade of a screwdriver and carefully scrape all the hard water build-up off the top of the rubber seal being careful not to puncture or tear the seal. Then, soak the seal down with Pledge, just push down on the seal and spray it. Let it set for five or ten minutes and then turn on the water and test it. Your commode should retain water . Dometic used to tell you this in the commode literature, back in the good ole days!
I've done this for years, and I also found it made the cleaning of the commode simpler...so use it often.
Happy RVing!
Joella