It sounds like you have a ground/neutral short in the MH or the adaptor. Unplug the MH from everything and use a multimeter to check for continuity between the ground bar and the neutral bar in the breaker box. They should not show any continuity.
If they do, disconnect all of the neutral wires from the bus and reconnect them one at a time, rechecking for continuity. When you find the wire that has continuity with ground, you know which circuit to check. Do the same with the ground wires (separately). Now you have identified which ground wire is shorting with which neutral wire. All you have to do is trace them to see where they are touching.
Terrydactile wrote:
There have been a good many issues such as yours that were caused by the 30 to 15A adapters being faulty. Try using a new one.
That is easily tested for. Just plug the adaptor, by itself, into the GFCI outlet and it will trip.