If you are buying a new or expensive coach, a Montana LLC is probably the cheapest option as it will save you the sales tax (the main reason anybody registers through Montana LLCs). If you are buying a used / cheaper coach which doesn't have as much tax costs using a South Dakota mailing service works very well and is much easier to setup and maintain. I think it's 3% tax on vehicle registration, no inspection on registration or annually (so you can register it there without having to actually go there) and the mail forwarding company can handle the registration and renewals for you.
I'm a Canadian and I used Americas-Mailbox (
http://www.americas-mailbox.com/) a few years ago to register a mh we were leaving in the US for the winter (I'm Canadian and eventually registered it in Canada but couldn't register in Canada at the time because we weren't driving it to Canada until the following spring (In Ontario, you can't register the vehicle without it actually being there because it has to be inspected). There are several different companies that do this in SD.
Different states have different residency requirements for registering a vehicle so you might not even be able to register it where you buy it (e.g. I wasn't even able to get a temp permit in AK because I wasn't a resident or US citizen (And I even have a US SSN). They told me I had to get the motor home towed to Canada)
