Hi and welcome to the forum.
It's not so simple as just deciding which state is cheapest to register your vehicles in. In most cases, you need to register where you are "domiciled"... meaning where you live, vote, see a doctor... etc.
If your plan is to hit the road full time and sever ties with California, then you should do a little research on the best state for you to domicile in. Searching this forum will give you a lot of reading material. We chose Texas. The Escapees RV club has some great information about it on their website.
As a California native who sold my home in NorCal and hit the road full time... I have some advice. If you are really leaving California, take the time to fill out the forms requested by the Franchise Tax Board and the DMV telling them you are leaving. They badgered me for three years wanting me to file a tax return with them after I left.
Best of luck.