The most popular states for fulltimers to select as their domicile are Florida, South Dakota, and Texas. None has an income tax, each has reasonable vehicle registration fees, and each has several mail forwarding services. Probably even more importantly, these states allow the use of a mail forwarding address as a legal address for establishing domicle.
However, there are differences among these 3 states, so which one is the best for YOU will require some research on your part to determine which will be best for your particular circumstances.