Correct me if I'm wrong, but I don't think the in-vehicle "ground" means
much to an object or person outside the vehicle. Rubber, as in tires, is a
pretty good insulator, and if the hot side of the circuit is trying to find
ground, it will do so more easily through a person or whatever than through
the tires.   A real ground, for 120 v.  home wiring, consists of a 6' copper
coated 1/2" rod driven at least 5' into the earth, as well as ground clamps
to cold water iron or copper pipes. Unless you're going to plant a true
grounding rod wherever you camp, you don't have a true ground. What effect
this has on GFCI's I don't know. But I can imagine they don't function they
way they're supposed to.