Man, what a puzzle. do you have another high current device you could plug into the GFCI to see if it's a worn receptacle. Maybe a portable heater or similar?
I hate troubleshooting from both ends at once but these are the most likely, IME.
The next may be to get a meter in between the neutral and ground of the trailer when plugged into a non-GFCI receptacle and see how much current is appearing. A good meter will be needed, one that measures in ma and possibly, one that will hold measurement in memory.
