I had this sort of problem when staying in a small park in the Seattle area that only had 15 amp GFI outlet at the space I was in. The rig initially worked O.K., but tripped the GFI the next morning after I used the microwave. Here's what I suggest you do to find the potential electrical leakage problem.
Do your best with a meter to verify that your trailer chassis is connected to earth ground.
1. Turn off all trailer Circuit breakers.
2. Reset the supply GFI
3. Turn on the trailer Main Breakers - Did the GFI trip?
4. Turn on the individual breakers one by one, checking each time if the GFI trips. It will be handy to turn on a lamp, or device with a power indication as the first individual breaker, to know when the supply GFI trips.
5. When you turn on the breaker that trips the supply GFI, you have found the circuit with the electrical leakage problem device.
For me in Seattle, the leakage problem was due to moisture buildup in the microwave from food steam. Drying out the microwave got rid of the problem.