Out of state license and storage.

Vehicles are generally titled and registered in the state where they're garaged. If you do as you say, there is nothing wrong with titling and registering in MT. And, I don't see any need for you to co-own it with another person. If CA ever came after you for tax evasion, proof that the RV has never been in CA, has only been fueled and used in any state other than CA, inspected in MT and in compliance with all MT requirements, should be more than enough to stop CA from taking further action.