[BB_TX]

First thing I would so is disconnect the power cord from the trailer and then plug it into the outlet and see if the GFCI trips. If it does not trip, that should eliminate the power cord as the problem.

Then I would turn off all breakers inside the trailer and reconnect the power cord. If it trips when you connect the power cord, then there must be something between the power cord and your breaker panel.

If it doesn't trip, one by one turn each breaker back on giving enough time for each to see if the GFCI trips. If one trips, that should narrow it down to a specific circuit.

A legitimate GFCI trip normally means there is a connection between neutral and ground somewhere in you system.