Currently, my fresh water tank slowly backfills from the city water supply (takes about 10 days) due to a faulty check valve in the fresh water pump. So, there is city water pressure at the output side of the fresh water pump.
I would think it would be somewhat simple to build a bypass around the fresh water pump, similar to what is used to bypass the hot water heater when winterizing. Whether there is room to fit the additional plumbing or you have sufficient access to do this would vary on your model.
First noticed the problem when I saw a trickle of water coming out of the fresh water tank filler door. Full hookup campsites, didn't have the tank monitors on. Funny thing is that I just weighed the trailer a day before discovering the problem and was wondering where all the extra weight came from.