A GFCI plugged into a GFCI is unreliable.
It may work; but then again it may not.
However in your case I think you have a ground fault in your wiring somewhere and the GFCI is properly responding to it.
The only way to trouble shoot this involves opening all the breakers and removing the circuit Neutrals from the buss; leaving just the incoming Neutral and incoming (main) circuit breaker "ON"
If the GFCI still trips, the ground fault is in the wiring connections from the power center to the garage.
If it is stable, then connect ONE circuit and related Neutral at a time until you get the trip. Then troubleshoot that circuit till you find the bare copper wire (ground) that is touching the white neutral in the junction box.
Have you tested your coach for a "hot skin" condition?
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
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