You need to check to make sure something isn't on inside your RV. If my refrigerator is on auto (switch from gas to electric) when I get home from a trip the GFI trips when I plug in.
If you can't find anything on....try flipping the main breaker in the RV to OFF and then plug into shore power to see if the GFI stays on. If it does then flip the main on to see what happens.
Another possibility is a ground problem somewhere in the RV. Ground isn't connected properly on some circuit or a short somewhere...