Since the cause is running the pump when city water is on and it's so easy for that to happen when others are in the trailer it might be a good idea to install a check valve ahead of the pump.
Valterra makes one that merely screws on the male threads of the inlet filter and then the barb fitting that came off the filter screws onto the check valve.
About $12 at Amazon
If installing one just make sure the threads are well sealed so air isn't sucked in. Install one and no more worries about overfilling the tank ----- period.