Turn off all your breakers in the RV panel. Plug in the GFCI shore power pedestal. IF it does NOT trip, then your shore Cord/Transfer is OK. Turn on the 50 or 30 amp MAIN breaker in your breaker panel. If it does NOT trip the GFCI, then turn on 1 Breaker at a time until the GFCI trips. Once you have determined which breaker when turned ON trips the shore GFCI. either run your APU and then determine what outlets/appliances are ON that circuit. Then trace and disconnect each appliance or circuit 1 at a time until you find the problem. Doug