Moisture in one of the plugs is probably the issue. Check the outlets where water might have run into them.
If they seem wet, disconnect all AC power from the trailer, and shut off your inverter if you have one. Let the outlet dry out completely, then try resetting the GFI after restoring power. If that does not fix it, get knowledgeable help.
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.