I've bought three motorhome/toterhomes over the years from Indiana.
Some states work in conjunction with California. For example, last year I bought a new Intech trailer from a dealer in Ohio. The trailer was manufactured in Indiana. I drove from So Cal to Indiana to get the factory tour and picked the trailer up while I was there. But because the selling dealer was in Ohio I had to use Ohio's rules. I was given an Ohio temporary permit, and I paid Ohio sales tax and a small fee which is less than California's. When I took the paperwork to the DMV all they charged me for was the balance remaining and the smaller license fees.
When buying in other states some will not collect sales tax from you, but you are still responsible for paying it to complete the California title and registration process. In order to move the motorhome the dealer in the state you are purchasing it from will work with you to give you a temporary XX number of days permit to drive it. You can't legally drive it without some sort of issued permit. If the dealer isn't able to cooperate on a permit or tags there are services that will do it for you. My best advice is to ask the dealer first. It's likely that you aren't their first customer from California and they will handle it or guide you. If they can't help you, call the California DMV and have all the information ready like VIN, type of rig, etc., and they will guide you. Also, call your insurance company to make sure they understand it's an out of state purchase and you want to make sure they have full knowledge of what you are doing and that you expect to be insured.
The last out of state motorhome purchase we made was over 10 years ago, so I'm not up on the current 49 or 50 state legal smog requirements or labeling on new motorhomes. Again, the selling dealer should be aware of any pitfalls, but it never hurts to do some checking yourself. This link from the California DMV doesn't mention motorhomes specifically, but it does have some good information.
https://www.dmv.ca.gov/portal/dmv/?1...t_facts/ffvr29
Good luck.
Randy