We started with two kittens 12 years ago. Litter box behind the driver's seat. (rear seat removed)
As kittens they were hard on the camper. Bent the mini-blinds and scratched the veneer off the wall board.
By the way, kitten poo stinks far worse than when they become adult.
And they always wait to poo until everyone is inside the cab on the road with the windows up. . .argh! I've actually had to stop the truck and clean the litter box right away.
They were both outdoor mousers and performed a useful task at home - keeping the rodents a bay. Over time, we lost one to a coyote at home. The remaining guy still travels with us. He's an old hand and knows exactly what the truck and camper is about.
He long ago discovered that the space between the camper and back of the truck is wide enough to escape through. His refuge when outside is on top of the rear axle.
Campgrounds with kids and dogs are best. He'll stay inside. But if we pull into a boondock area, he will often scout the area from the windshield. If it looks peaceful and there's good cover nearby, he'll hop out almost immediately.
It's a risk having him outside. He knows how to hop back into that space behind the cab or hide above the rear axle. Even so, we keep him hungry toward evening so that he'll come when called. And then he's locked up for the night. (Although he would prefer to be out cruising.)
During early years, big trucks passing on the highway would scare the bejeezus out of him. But now very little scares him. It takes time but they adapt to almost any situation.