Time to start pulling wiring plates on outlet boxes / microwave / etc to check wiring. It would appear that somewhere a wire maybe up against something it shouldn't be. Outside of the trailer frame, it is unlikely that any other building "components" are normally grounds so they are "floating". Somewhere something "hot" is touching one a "building component" making it "hot".
Try isolating the 120 distribution at the breaker panel, one breaker at a time until you lose the 120 "bite"... possibly this will point to the culprit circuit.