This is the publication to reference when purchasing a vehicle out of state.
Pub 52There is much information in this thread that is in error or not correct. Pub 52 will give you the way to avoid paying California taxes, note i said avoid not evade as tax evasion is illegal.
On a $150,000 RV the tax at 7.25% (I think the lowest for CA some as high as 10%) is $10,875.00.
To answer the question why buy out of California? To save the sales tax on the purchase. If you buy a vehicle in CA you will pay the taxes for where you live if a CA resident or you will pay the taxes for the jurisdiction where you buy if you are not from CA. If a CA resident that lives in CA but properly buys out of state the vehicle will still be registered in CA through the DMV with the tax clearance from the Board of Equalization (BOE) if the RV does come back to the state. Note the DMV has authorization to collect for the BOE.
JimR