I have been RVing with my kitty for much of the past 2 years. He was 11 when we started. It's been much easier than I expected, and I love having him!
He's indoor/outdoor, and he and I consult on when and where is safe for him to be outside. Boondocking is best for that. RV parks full of off leash dogs, not so much.
Definitely get the cat used to the RV, and thinking of it as part of their territory, before you go. I knew I was there when my kitty stopped letting his best friend cat come in the van, because it was HIS. Driveway camp with the cat too, before you leave. I did this for several weeks, while packing up my S&B.
You can get a harness and leash for a cat, but they can get out of even the ones that guarantee no cat can escape. In about 5 seconds :B. My cat is very accommodating, and will stay on the harness if I ask him too, but he feels safer off it so I don't use it unless a park requires it. When he gets spooked he runs TO the van. That is part of why it is important that it is his territory.