Our cat Scruffy already has inside access to the front pass-through where her litter box is kept. If we're going to be around the campsite I hang her patio in the hatch opening. Scruff likes to come...
We put a halter on Opie and tied him up outside with us. He had never been on a halter and leash until we returned to camping but he loved it. He would go to the door to tell us he wanted out and than go sit on his leash so we knew that he wanted out. We did not allow him out on his own altho he tried to tell us that he could.