Our traveling companion, Archie:

On the beach at Fort Flagler State Park (near Port Townsend, WA):

He does just fine in the camper, and rules the roost when we are out camping. He just makes it his domicile and allows us to visit.
I have a pet bed in the backseat of the truck where he rides. Except during evenings, then his nocturnal nature kicks up and he sits on the center armrest, giving directions to all the other vehicles around us. I have done many drives home alone - DW asleep in the passenger seat, and Archie snoozing in the back. He does wear a harness and has a leash clipped to a seat belt when in the truck - animals can be unpredictable, and the last place I want to have that happen is on the road with the window down or something!
We have been very careful to NEVER let him exit the truck or camper on his own. We ALWAYS carry him across the threshold. This way he has never learned it is a place he can go on his own. In fact last year we had the camper door pop open in the middle of the night, and I didn't realize it immediately. What woke me up was the cat making a beeline for the bed and trying to hide under the covers with us.
Years ago we took our previous cat, Oscar, out tent camping. Middle of the night I heard the tent zipper and he was gone. Here I was at the State Park at 3am in my skivvies with a flashight saying "Here kitty, kitty". A dog barked in the distance and zoom came the eyes out of the darkness heading right back to the tent!
He is a good companion.
-Eric