Several years and another RV ago, I had a case of GFI circuit tripping every time I plugged in the RV, but it wouldn't trip a non-GFI circuit breaker...after much work and frustration it turned out the water heater element was shorted. This was caused by plugging in to shore power with the water heater on and empty (this was my first RV and I didn't know the water heater even had an electric element in it)...try disconnecting the wire to the water heater to see if the gremlin goes away...if it does, replace the element in the water heater