taxes and registration
Many years ago, all of the states agreed that the state where a vehicle is registered is the state where all taxes and fees will be paid. And to be legal, that state must be the state where the unit is garaged, or in other words, kept the majority of the time. If you plan to go fulltime, then you can easily choose any state as your legal residence, as long as you meet the requirements for residency in that state. Probably the most popular states for residency of fulltimers are Florida, S. Dakota and Texas. That is true because all three are easy to qualify for residency and none have a state income tax or excessive registration costs. Another way to save tax money would be to form a Montana LLC to own the RV and register it there. But that too only works if you are going fulltime. Since you don't say if you plan to go fulltime or not, you do need to realize that all states require those who live and work within their borders, to have a driver's license, vehicle registration and various other things in that state.