Though in SOME cases they wire the Converter on the GFCI there is no reason to do that. So the converter should have NO Effect on the GFCI inside the RV. none zip. nada..
But the GFCI in the garage is a different animal.
Test procedure
Step one turn off ALL Circuit breakers inside RV
Step 2 plug in RV. you should hear the CLUNK, GFCI shoud hold.
Step 3 Plug a 120 volt Drop cord light (Trouble light) into other half of duplex outlet you plugged RV into and place the light where you can see it INSIDE the RV.
Now turn on the MAIN breakers inside the RV.. Light remains bright. GOOD
Turn on the 1st Branch if light remains bright turn on the 2nd, and 3rd and so on till light goes out. TURN that breaker off Reset the GFCI (light back on) and continue with the NEXT breaker till you have identified the breaker(s) that trip the Garage GFCI.
NOTE what they power
Good chance you have some thing like water in the patio outlet box due to a failed cauling bead. This will trip the RV's GFCI and might do the garage as well. but the "Which breaker kills it" test will find it.