Without the battery in place to act as a voltage regulator or not having the 12 volts to run the refer makes me think you have a 120 ac volt problem. PERIOD.
Without 12 vdc the refer will not work. When the MW goes on it starts burning a bad connection and makes a contact because it is expanding. cooling down and the connection goes away. Start looking in the 120 vac box for loose or melted wiring. Or the cord and plug where the camper is plugged in (not so much) sounds like a loose neutral wire probably in the camper main fuse box. Don"t think anything is wrong with the convertor or the 12vdc side of it.
