Apples and oranges.
In an old home without grounding, GFCI outlets will still trip because water and gas pipes buried underground provide an earth ground source to divert any ground fault current leakage around the GFCI outlet back to the power source to create the imbalance.
With an RV where the generator neutral and ground are not bonded and the RV and generator are not grounded to earth there is no path for any fault to leak current around the GFCI outlet. (If you know of a path please tell me.) That is why the National Electric Code section for RV generator power statement posted on page 1 of this discussion states the requirement for bonding and creating a ground-fault path. That is further supported by the OSHA statement on page 2. Basic principle of electricity is that a closed circuit is required for current flow.
If I am wrong please explain why.