I also second Steven's recommendation of the Escapees RV Club which has a lot of support services for the fulltime RVer.
Among fulltime RVers, South Dakota, Texas, and Florida are the most popular states to select as a domicile state. None of them have an income tax, vehicle registration fees are reasonable, and, probably most importantly, they allow the use of a mail forwarding address as a legal address for such things as driver's licenses, vehicle registration, etc.
There ARE differences among these 3 states, however, so some research on your part is required to find out which is best for YOU and your circumstances.