I'm sort of reviving this thread because after adding a second circuit and some outlets in my Scamp
I was having the same problem as the OP was. Tripping GFCI plug used as shore power in garage.
Thanks to the original post having brought the subject up, and Jon in post #8 providing a link to some troubleshooting information in another forum. RV.Net Open Roads Forum: Tech Issues: Electrical
Along with the discussion in this thread by folks that really know this stuff. I was able to track down the problem. I know a bit about wiring but GFCI outlets were just starting to become a "thing" when I stopped working on this sort of stuff.
Top clue was tripped garage GFCI plug with circuit breakers in scamp
off, meaning neutral / ground fault (since no power to scamp
wiring with circuit breakers off) Then it was a matter of disconnecting all the white neutral wires to confirm not tripping without neutral connection. Then just a matter of re-connect each one to shore line one at a time until plugging in at garage caused a trip. Turned out to be an old outlet I used with own box that I had installed with the wires placed so that when cover was tightened it crushed and connected the white/green neutral/ground wires together. Opened up, shifted wires, plugged in, no trip.
I am going to replace that old part, started thinking about it and that part had to be well over 20 years old. Been sitting in parts box all this time waiting to be used and now I'm going to pitch it. Makes me sad. On the other hand ..... Our members are just darned helpful, makes this forum where I did the search rather than Google.