Look at the outlets in the bathroom, kitchen, and outside. All are probably on the same GFI circuit and if one of them has gotten wet, that could do it.
If you don't find anything, switch off the circuit breaker that supplies power to the GFI outlet, or unplug the trailer from the power pedestal. Remove the cover plate and pull it away from the wall so you can inspect the wires on the back side. If press-in wires, you might want to use pliers to make sure the wires are fully inserted.
Again, do this with the power off.