Have you tried resetting the GFI while the coach is still plugged in -- like blw2 mentioned in post above, that may work. It's worth a try.
If that doesn't do it, I'd try to find what is leaking by turning everything off and then start by adding one thing at a time. I'd start with extension cord alone first, then add adaptor, etc... Inside RV I'd start with all breakers off and see if that prevents tripping. It's a difficult job for sure.
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