It is a buyers' market for RV's and easy to save 25% compared to a new vehicle and you avoid having to take a RV back to the dealer for various items that might need fixing during the first months of use. A lot more to do wrong in an RV than a car.
Lower purchase price is going to be more important than saving on sales tax. Certain markets have a great many more used RV's from which to choose, like Phoenix, Arizona or even southern California. And advantage to California is that buying extra items for the RV there will be many more stores with RV related items than in any other area except perhaps Florida.
Of all the states I have traveled across the one I would not have minded missing out on at all was Texas.