Months later - here is the update.
After the bathroom GFI, the circuit goes to 2 outdoor outlets, then outlets in the living room, then 2 outlets in the kitchen. I turned off the power to the rig and tested the continuity to all the plugs by shorting them together and measuring resistance. All were were connected. So I removed the outdoor outlets and found one bad connection. The wires in the outdoor outlet were the kind you just insert into a hole - not tied down the nearby screws. So I removed a loose connection and screwed it down. That was the culprit.