It seems to me there has to also be a connection of hot to ground to put a substantial voltage on ground. Perhaps a defective load somewhere in the trailer or a misconnection in the wiring. Tripping the main breaker to prevent the hot side of the AC power from going anywhere inside should stop it. If not there has to be a hot to ground connection in the cord or wiring before the main breaker. And probably one of the other breakers will stop it so you will know where to look for the bad connection.