OK
In some cases the wiring in the Trailer can cause the GFCI in the campground hookup to trip as in your case. This can be caused by a number of problems, many times moisture or corrosion on the power cord can cause this problem. During normal operation the current supplied by the hot wire returns through the neutral wire. If a different amount of current returns to the neutral wire the GFI read the condition as a ground fault and trips. With everything being checked already or replaced I would next look at that cord. Some cords will come apart and others are sealed. I would also inspect your battery, make sure there is no corrosion and everything is tight, try to follow the cables and make sure it has not been rubbing in any spots. Sounds like it could be the cord.
Thank You! Please hit ACCEPT for my efforts, Positive feedback and BONUS welcomed!!