There is one hole in the gentleman's story about selling his home in NY and then becoming a full timer....what state was he a resident of when he bought his rig with his Montana LLC? Was he a Montana resident? Did he have a Montana driver's license? Did he still have his NY driver's license?
Declaring oneself as being a full timer is great but you still need some sort of personal address. Many use SD or TX. What he did with Florida sounds like it is legal, but he may not have been legal when he purchased his rig with his Montana LLC if he was still using a NY driver's license. We have been fighting with NYS for over two years on the issue of whether or not one of our clients was a Florida resident rather than NY. To give you an idea of how much of a challenge that can be, try to prove you've NOT been somewhere. Your NY/FL/MT friend may be in for a visit from the fine folks at NY Taxation and Finance before too long if he still had his NY driver's license at the time he bought his motorhome.
Now if he claimed MT residency and got an address in MT and changed his license to MT then he would probably have been legal. As our friend from Billings has said, know the law, comply with the law, and document your compliance. I'm not sure this guy met all three conditions.