As far as I know, a GFCI is not a circuit breaker and will not trip unless there is a ground fault. Therefore, it should not be used as overload protection on a circuit.
You are correct in assuming that the smaller circuit will be exposed to additional power, but if you are supplying power to your rv, then any and all circuits will be protected by their appropriate circuit breakers in the electrical panel.
The smaller cord will get warm/hot if overloaded, but if it fails, I.e. melts, then it should create a direct short and trip the 30 amp breaker.