If she is plugging into a typical 15 amp garage receptacle, ask how much extension cord is being used and what was powered on when the magic smoke appeared.
If she had an electric water heater powered, the AC on or any combination of high draw appliances, the draw total may have been too much for the trailer.
At that point, the converter trying to charge depleted batteries is seeing a voltage sag but an amperage spike. First diagnostic test for a situation like that would be to measure voltage at the battery while 120V is available. If 13V+ is not present, there may be a tripped circuit breaker or blown fuses on the converter.
My experience is that once magic smoke has been observed, there is a loss of either wiring or a device. Sometimes, a guy gets lucky and a tripped breaker or blown fuse saves your bacon.