I have an outbuilding that's fed from the main panel in the house but has its own independent grounding electrode. There's a neutral-ground bond at the main service entrance, and another at the outbuilding. The grounding points are maybe 200 feet apart. I ran a wire between the 2 buildings and measured the voltage between the building grounds which was in the 0.4 volt ballpark. Switching on a big 240 volt load in the house changed nothing. 120 volt loads cause a change. Specifically, a 1/2 HP air compressor changes the ground potential difference between the buildings by 0.2 volts, higher or lower depending on which 120 volt leg it's powered by. The load on the neutral is eventually carried through to the grounding electrode via the neutral-ground bond, causing its voltage to be pulled up a bit from the actual ground potential. Back to the rv situation, I'm suggesting that the shore power's safety ground could legitimately be at a higher potential than the actual no-load ground potential. How much depends on the neutral current, resistance from the bonding point to the grounding electrode, effectiveness of the grounding electrode, and potential differences at different points on the ground. All theory and speculation, none of which establishes that the OP's rig is now correctly grounded.
