The cheapest state to buy and RV sales tax wise is South Dakota, its about 2 to 3 %, however the easiest is Texas and thats about 7 % sales tax.
All really depends on if you are planning on shipping straight out of the US or going to use it there.
We have a 5th wheel thats Texas registered and have a bank account and drivers licences for Texas, we use it 6 to 9 months of the year and then when back in UK and Europe we store the Truck and 5th wheel near Dallas for £25 a month in covered storage.
We are also at this moment in the process of shipping a 2008 Coachmen Concord 300TS back to UK from Florida, deposit has been paid and we are waiting for the wire transfer to complete, as its leaving the country we do not have to pay any US taxes.
Another option is Montana, if you set up a limited liability company (costs around $400) to set up, then you can buy an RV with no sales tax to use in the US.
Hope this is of some help, if you need any more info please ask, we are into year 3 over here now and are pretty much aware of the rules and regs in most states.
The answer is really longer than I could post in a message, but Kevin, Hazel and John are on the right lines. However, here's my somewhat simplistic my take on it:
The main reason folks use a Montana LLC is because it allows you to register a vehicle there, irrespective where it was purchased; Registration fees are much lower in Montana than many other states, which is obviously more significant on higher priced RVs, but an LLC is not without pitfalls. e.g.:
Folks entering California have been challenged because, since their coach was registered to an LLC (the Montana plates are a dead giveaway), it was considered a "commercial vehicle" and required a permit to drive in California. Since they didn't have a permit, they were fined. The FMCA mounted a challenge, and provided their members with a letter explaining it was a misinterpretation of the law. (Try explaining to an official with a gun on his belt that he doesn't understand the law.)
When I recently walked through the options with some friends who took delivery of their (very nice, second hand) coach last week, they opted to pay the California registration fee than go the LLC route, because the savings weren't worth the potential drawbacks.
Avoiding (or minimising) sales tax can be achieved (legally) in one of two ways:
Buy in a state that has no sales tax, or one with a low rate. However, buying in a zero tax state doesn't mean you'll necessarily get the best deal there.
Buy in one state and take delivery in another, but there are likely to be restrictions, depending on the states involved. Currently in California, this option requires that an RV or boat be kept out of state for 91 consecutive days in the first 6 months of ownership. The state also audits people and looks for proof that the vehicle or vessel was bought ostensibly "for use outside of California".
We've used different options for boats and RVs, but I continue to shy away from the LLC route. Montana is starting to ask a lot of questions, and I prefer not to run into someone in a state uniform who got out of bed on the wrong side that morning.
Obviously some of these factors are more significant for people living in the U.S. than for visitors and, in some cases, might be more significant for people owning property here.
Bottom line is 'how long is a piece of string'. Or, as we say in Wales, your mileage may vary.