GFI tripping when plugging in to charge batteries

There is nothing wrong with your trailer. There is no ground fault. There is 2 ways to fix your problem. Dealer not required.

Your trailer has a GFI circuit in it. Sometimes 2 GFI's in series don't play nice. One way is to turn off your GFI 120 volt circuit breaker in the breaker box of your Trailer. Not the outlet in the bath. Everything will work except any outlets within 6' of a water source.

Option #2 is to plug into a non-gfi circuit at the house.