Bad GFI's are more common than we think. They are very sensitive to the current flow and can default. If nothing else is obvious, it may be the GFI. We bought a new house once that was located not to far from RR tracks. Everytime a train went by, it tripped the GFI thru ground vibration. Weird. Hope you have figured it out by now.
Barry & Sue Miller("extended" RVer's, 6 mos + per/year) combined trips.
04 Journey 39K, C-7, 330 hp, towing: 02 Dakota 4X or 2013 Ford Edge AWD, Aero 5050XL, ReadyBrake.