South Dakota, Texas, and Florida are the most popular states for fulltimers to use as a domicile. None of them have an income tax, vehicle registration fees are reasonable, and they allow the use of a mail forwarding address for establishing domicile. While Texas does have an annual vehicle inspection, you do not have to return to the state just for an inspection, but it does need to be done within a certain time from once you return to the state. There ARE differences among these states though, so some research on your part will be necessary to determine which one is suitable for your circumstances.
Once you've determined which state is best, the next thing to look for is a mail forwarding service in the state. If you select Texas, there is none better than that provided by the Escapees RV Club,
www.escapees.com (a club that, IMO, all fulltimers should belong to).
In South Dakota, there are many mail forwarding services, many of which advertise in the various RV magazines. We use Alternative Resources in Sioux Falls and have had very good service from them,
www.alternativeresources.net.
I'm not as familiar with the mail forwarding services in Florida, except I know Good Sam's mail forwarding is located in that state and I *think* Passport America's mail forwarding service is, too.