Your ground fault or short has nothing to do with the 12v fuses so don't bother with them. Sparking sounds like a short to ground. If you have the camper main off when it sparked it has to be in the cord.
You don't need a heavy extension cord to trouble shoot a ground fault or short. Even a light duty one will work if you have it.
You either have a neutral touching ground causing a ground fault or a hot wire touching ground causing a short. A direct hot to neutral short would not trip the GFI but would trip your circuit breaker feeding the outlet.
