UPDATE
I took her to a repair shop I trust yesterday. Got a call later in the day that they were 99% sure they found the problem. Two of the house batteries were dead or close to it. He explained that when plugged into the 50 amp service, the inverter was trying so hard to charge the dead batteries that it caused havoc with the whole 120v system. All he did was to replace the four 12v batteries with four 6v GC-2s and everything works fine.
I watched as the park’s service guy checked voltage on both legs of the pedestal last week so it wasn’t a park issue. I hereby give up my electrical guru lapel pin.