Our cat gets car sick, very fast, and it's very messy. He runs at both ends, and that's absolutely no fun at all when you're dragging 35 feet of train behind you and you just can't whip over to clean up a stinky mess.
After MUCH trial and error we found a solution that works very well for us.
We put a towel in the bottom of the shower. We put the cat litter box in the shower. We put the cat in the shower. We then lock the shower door. Cat cannot get out of the shower.
Once we start moving, he can vomit and poo in the litter box (which he does) until he's past it all. It takes about 30 minutes for everything to be finished. So usually on our first stop, we then bring him forward into the cab of the truck. As long as he stays on the passenger lap, he's fine. Turn him loose and he'll crawl under the back seat and vomit again.
We now, after 10 years, have him accustom to traveling enough that, sometimes, we can start the day with him in the cab of the truck, as long as he is sitting on the passenger's lap and covered with a towel or blanket, he's fine.
This simple method took years for us to figure out, and we tried everything.
Also, we have a wireless indoor / outdoor thermometer. We put the outside sensor in the bathroom and the unit in the truck. When temperatures begin to rise and the inside of the trailer hits 90 degrees, we always bring the cat forward into the cab.
When we stop, we always put the cat into the shower immediately, so he's confined in a small space and easier to retrieve when we return to the truck to continue our journey. Of course, when we reach our destination for the night, he's free to roam the camper.
By the way, the cat loves the camper. We use it 24x7 from end of March through December in the drive way or at home when not camping. Because we live in the country, the cat runs free via a doggie door in the house. He spends more time in the camper than the house, same as us. He sleeps in the camper every night with us, and will often sit on the trailer steps waiting for us to open the door for him. He knows this is "home". And when camping, he knows we are somewhere different and is actually terrified to step outside the door. He knows the difference. Except for his car sickness, he's a pretty good kitty.