The standard adapters should connect both 50A hots to the single 30A or 20A hot and neutral to neutral. Given that a upstream GFI should not trip.
I don't understand this! Isn't there 240 volts from one hot to the other? Surely connecting them together would trip both circuit breakers.
I would expect the adapter would connect to only one side of the double 50 amp supply. There ought to be a 30 amp breaker in the adapter but it isn't really needed because the RV has one already.
It would be interesting to understand how an adapter causes a GFI to trip.