Short answer is Vermont. Sorry to be a downer, long answer follows.
Living in the Northeast and working in government here, I can tell you that your registration has to be in the city and state where the vehicle is garaged. I saw a reply saying to use Florida. Sure you can register it in Florida and not pay taxes on it but that is illegal. In my state they have started to aggressively go after people that do this. When you are caught, they can go after 3-6 years of taxes plus interest, penalties, and administrative fees. They also will put a lien on the RV on top of it all. When all is said and done they will be pretty close to owning your RV.
Municipalities are hiring outside firms to track this stuff down now and use all kinds of records including DMV and insurance data. Vermont is leading this charge in the Northeast.
An insurance company is well aware of "vacation" homes. It may actually cost you less. You just have to ask yourself if it is worth the risks.
2015 FR 29V Vengeance Toy Hauler
2015 Honda GL1800 F6B
2009 Dodge Ram 1500, Total Load Control air bags, Blue Ox Swaypro.
Technology presumes there's just one right way to do things and there never is.