Disconnect from shore power, turn all breakers off. Remove the GFCI and expose the wires so measurements can be taken. Plug back into shore power. measure voltage at wires. Turn the main 30 amp breaker on and measure. Turn individual breakers on and measure. While you are turning on individual breakers you should have power to the GFCI wires. That will indicate a miswired or faulty GFCI. If you don't measure any voltage at any of the above steps, you have a wiring or circuit breaker problem. Mark on the load center which breaker controls the GFCI circuit.