Cheap states to register in are states like NH and TN that have no income tax.
I can register my vehicles and then renew the registration online annually with TN. Insurance is pretty inexpensive for vehicles too.
Beware the RV thing though if you're the kind of person who bounces off a low balance in your savings. If the rear axle falls out of it, or the trans takes a crap, you're going to need a couple thousand to have it fixed, if you can't do it yourself. Even then, you're looking at a few hundred for parts if mechanicals die. It's conceivable that the rear axle, driveshaft, transmission, and engine could all take a crap at once or in the same month. $$$
And, of course, you have to maintain it. Plumbing and electrical need to be maintained and repaired, batteries have to be replaced with some frequency, and then there's routine maintenance like oil changes and tires, etc.
Make sure to have a safety cushion made of cash for RV repairs and maintenance.
"If they would have made this 15 years ago... Well, I would have killed myself to death actually. But what fun while it lasted.