There used to be a company called PSRV, they sold a 90 dollar box that did magic, real magic,, I can tell you how it worked with a couple line drawings
Original circuit to rear A/C in many motor homes
Shore-Breaker box---(optional switches)----------------Air conditioner
new configuration
Shore-Breaker box---(optional switches)----Socket/Plug--breaker--Air conditioner
The 2nd breaker is 20 amp, same as the one in the park power box so if you trust the park breaker you may omit it, That said, you should include it.
This completely removes the rear A/C from the motor home power wise, since it is independent, no common anything (Save for the 12 volt control lines) the GFCI is happy as can be and lets it work without issue.
Additional feature: When I needed compressed air on the SERVICE side of the RV,,, Fired up ye old Onan, plugged in sears Huffer Puffer (150 PSI pancake compressor) and had what I needed, no need to pass hose under coach.