1. Turn off water pump and city water
2. Put on a pair of disposable gloves.
3. Flush toilet, some water wills till come out but leave your foot on the pedal, leaving it open(it could be stinky smell coming from black tank so he prepared)
4. Take your finger or a q-tip, to where the valve will go on and around the rubber gasket, see if TP, poop or anything is stuck in the rubber seal that make the valve not close correctly.
5. Release pedal
6. Take off and throw away gloves
7. Turn water back on and put some in the bowl, see if holds it now.
If not, you can buy new gasket and it will require removing the toilet to replace the gasket, you can youtube how to replace the gasket for your model of toilet, or hire a RV tech who is willing to do it for you.
