the other possibility is that you have a ground/neutral bond somewhere in the trailer. GFI will trip if they find neutral and ground bonded together downstream of the GFI. this is also consistent with the operation with generator but not household powere. Generators have isolated neutral/ground, while house circuit has ground and neutral bonded at the panel.
It sounds like you may have a ground/neutral bond AND a loose neutral connection at the panel of the trailer.
with an ohmmeter, check to see the resistance between neutral and ground on your trailer by measuring resistance between the ground and neutral pins on your plug. (power disconnected of course)
It is possible that either the PO or you in replacing or working on the converter tied neutral and ground together in the trailer. If this is the case immediately rewire so ground and neutral are NOT connected. It is a NEC code violation, and a possible safety hazard that can lead to a fatal shock if hot and neutral are reversed at a hookup.
RV fridge 120V elements are known for developing a low enough resistance to ground to trip a GFI as are 120V heating elements in the water heater. Check them as well.
And the GFI doesn't care about anything running on 12V. It does care about ALL the things on the 120V side. Once you verify and/or fix any ground/neutral bond, turn off ALL the 120V breakers in the trailer, including the 30A main. ,plug it into the house. should not trip the GFI. Then turn on the 30A, then one by one turn on the 120V breakers in the trailer. Find the One(s) that trip the GFI and then see where the leakage to ground or ground/neutral short is. Fix them and you will be back in business.
