It takes some serious diagnosing to find GFCI problems sometimes.
1. Turn off all breakers in the RV. If the problem goes away, it is one of the hot wires leaking to ground. Turn the breakers on one at a time to identify the wire, then trace it. If the problem persists with all breakers off, it is not a hot wire leaking to ground.
2. Disconnect all neutral wires in RV breaker box. If the problem goes away, one of the neutral wires is touching ground. Reconnect each neutral one at a time to identify the wire, then trace it. If the problem persists with all neutral wires disconnected, it is not a neutral touching ground.
3. If the problem persists after #1 and #2, the problem is either the outlet, or between the outlet and the breaker box. That means one of the adapters, the plug on the RV, the point where the RV power cord is connected to the RV, or the ATS (if you have one).
It should go without saying, but I will say it anyway, UNPLUG THE RV BEFORE WORKING IN THE BREAKER BOX!