I keep popping the GFI plug when I plug into shore power

If the GFCI pops with the main breaker off, it could be a problem is the shore power cord, or could be a neutral-to-ground fault in any of the circuits beyond the breaker panel. To locate the problem, the only way I know of is to disconnect the neutral wires from their bus bar to isolate which circuit the fault is in. If you're ok messing with the wiring in the panel, ok, otherwise find somebody who knows what to do.