GeoPen, welcome to iRV2.
While you don't have to have your mail forwarding service located in the same place as your legal domicile, IMO it makes things neater if there is ever any question - like a question from your previous home state regarding taxes.
Texas, South Dakota, Florida seem to be the most popular states for Full-timers to establish their domicile because these states have no state income tax. I don't know about Florida, but Texas and south Dakota make it relatively easy to establish residency. They also have reasonable vehicle registration rates and insurance costs, but you will need to check those things out for yourself because everybody's situation is different.
As FlyingDriver mentioned, Escapees
in Texas and Alternative Resources
in SD are popular for mail forwarding, but there are others. Escapees service is legendary, but we went with Alternative Resources (and we have no complaints) because vehicle insurance costs in SD were much lower for us.
There are lows of pros and cons for each location. Scroll down to the Full-timers forum under "RV Life Styles" and do a search for "domicile" or "residency."