Someone asked about camper vans not too long ago, do a search?
When I got one I needed an adress for car reg purposes and insurance. You can get insured on a UK license, but make sure you have a new one.
There is a state caa tax sticker you need, some states ask for a smog test on emissions before you can get one, notably california. Might be worth buying somewhere outside Cali, and driving from there.