We lost one of our cats due to age last fall. Now my little guy, Archie, is an only child and loves it. We have taken him on weekend getaways three times now.
He rides in the back seat of the truck in his nest, usually asleep. I have taken him for vehicle rides his entire life. I won't say it is his favorite thing, but he knows what is going on and isn't a spaz. We live out in the country down a winding road with overhanging trees. He knows that means home, but gets confused when we are on a different, but similar road.
In the campground we just put him in the camper. He goes up to the cabover and camps out. His nest is up there and he will snuggle with us all night. One time we were at Cascade Locks in the Columbia Gorge. Fun little campground, right next to the railroad tracks. I like the trains, but Archie wasn't too thrilled. We came back from a bike ride to find him buried under the blankets as a little mound on the bed.
Yes, he has a harness and a leash. We do take him outside, but he is anxious to get back to the camper.
The best advice I can give you is NEVER let him go across the door of the camper (or door of the truck for that matter). ALWAYS carry him. That way he doesn't think it is place he can cross anytime he wants, he knows that he is always to be carried when he goes out the door. Sure, it looks cute to have him come & go, but I do not want him thinking this is something he is allowed to do on his own.
For camping cat amenities, I have a tall sided litter box. It sits in the shower (dry bath) and we are sure to keep the bathroom door propped with a shoe or something. The box comes out for showers obviously, but that is a short and temporary situation. A water dish, and that's about it. At home he is NEVER allowed to eat unless we are eating, so we don't keep food out for him - otherwise it would be feline-overeaters-anonymous!
My truck does not have a slider on the rear window, so going back to the camper while driving is not an option. Our trips have not been long enough to warrant an in-truck litter box, he is pretty good at simply holding it for an hour or two.
He is a good companion and I love having him along while camping.
-Eric