Start by turning off all breakers inside the rig.
Then try to plug into GFCI protected receptacle.
If it trips problem could be in the connector or there is a connection in the rig between the neutral and ground.
If it does not trip, turn on main breaker then each of the branch breakers to determine what circuit is creating the problem.
Something like a refer 120 volt element going to ground or a water heater element if equipped with 120 volt element.