GFI's go bad, old GFI were prone to nuisance tripping. If there are no problems found replace the GFI they are cheap. Check your camper power cord if the plug has been replaced take it apart I have seen a few with the black and white reversed the camper being a sub panel will function fine (but will not be safe) it will trip a GFI. With the wiring side facing you the black will be on the left white on the right and green on the top. There should be no 12 volt wiring inside the line side of a panel box. The white AC lines have a floating neutral it is isolated from the grounding system, the ground wires are landed on the box case grounding block then a ground AC wire (generally bare) is landed on the camper frame often along with DC grounds. If you stop and think about how the camper is wired you will see the logic it's all about never having a hot camper frame. Camp grounds if they can help it won't hire an electrician and lot of helpers do not know the difference between black and white!