Our cat hates to travel, especially to the vet, and she lets us know when we are on the road, but once in camp, she settles into the camper and does just fine. She is an indoor-only cat, but just in case, we keep her in a harness for the entire duration of the trip, and on a leash when we are in transit. We also have a child gate that we prop in front of the camper door (inside) when we have just the screen door closed.
We went through and rejected a lot of camper floorplans because they didn't have space for a litter box!