Hi Mark,
It's easy to go nuts, but here are a few other things....
Spare wheel studs for the car
Spare wheel studs for the camper
Spare shock absorber rubbers (especially for the trailer, if it has shockers)
Spare trailer bearing caps
An assortment of nuts and bolts (especially if you own a Land Cruiser....
)
A crow bar (yes, I'm thinking of you here yogi....
)
A couple of good, heavy ratchet straps