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JMHO: The two system heaters should have been the first thing he checked.
Each heater has separate wires to the control board , so they can be disconnected one at a time to check . Then CAREFULLY, with jumper wires plugged directly into the outlet to double check.
Over the years of production , Norcold used different hook ups for the ice makers 120 volt motor drive. Some years the drive was wired through the main board , and the GFI would trip if the ice maker needed defrosting .
Later models the motor had it's own plug direct into the outlet so it was easy to isolate. If the GFI trips with both heaters disconnected and you have an ice maker , it's important to isolate the ice maker's motor before you replace any other parts.