Assuming you have ruled out the converter, fridge and outside recept., pull every receptacle out of the wall that is dead after the GFCI, pull the rear cover off and ensure the hot, neutral and ground wires are properly "punched down" into the slots. Sometimes the factory workers do a really poor job of properly connecting the wires. I once found a wire barely hanging on. While doing this, also check to make sure the polarity is correct. I recently found one in our TT where the factory had the polarity reversed.
If you have replaced the GFCI and it's still tripping, you must have a ground fault somewhere. Are you positive you haven't missed a receptacle somewhere like say the microwave? It's not impossible for a romex cable to be damaged somewhere but that would be the last suspect on my list.
It's not for everyone, but you could take the disconnected hot, neutral & ground wires and wire them to an extension cord (by cutting the connector end off and using wire nuts) and plugging into a known functioning GFCI in your house or elsewhere and see if it trips. You could also pull a hot wire off the back of the dead receptacles one by one and reconnect the load wires at the GFCI and see if it still trips and also see which recepts. lose power and determine where each recept. is on that circuit. Pushing the wires back onto the slots in a "self-contained" RV recept. can be done with a flat blade screwdriver and/or needle nose pliers. Make sure to temporarily put a wire nut onto wires that would be live.