It all depends on the cat. Some didn't ever get over the sweaty paws, constant meowing and car sickness. When we had outdoor cats, one would come when called or just follow you, even in a new environments. The other would disappear for days, if it got out in a strange environment.
Current cat, picked from the animal shelter, is a big, black cat at 18 lbs (about 2 lbs over weight according to the vet). She calmly sits in the carrier, looking out the windshield when traveling, despite the noise of the diesel or the road conditions. We've always had her indoors and she hasn't made any moves to go outside whether at home or in the TC. We don't take the chance on the transfer between the truck and the camper and always use a carrier.
Did clicker training to get her used to going into and staying in the carrier, plus coming when called. Cat is really content hanging out in the TC. Comfortable with strangers coming in and out of the TC and greets them after they have sat down.
Cat box is in the shower. Food and water in the bathroom. We try and take a break after traveling 4-5 hours (when we are going long distances) so she can eat, drink and use the cat box, as she prefers to be with us in the cab and we don't have a pass through.