Any electrical experts out there?

Sounds to me like you may have a wiring issue in the trainer wiring. You said you tried the vacuum in all the outlets; I wonder if you also tried the microwave outlet? I suggest you plug the vacuum into the Microwave's outlet if you haven't tried that one yet. And/or try the microwave in the other outlets, too. There might be a short between the ground and neutral in part of the trailer's wiring that only leaks sufficient current to trip the GFCI when mucho amperage is flowing.

The OP's comment on 6/30 makes the most sense. Just a matter of tracking it down now.