Favorable states for full timers are FL., TX., and S.D. due to lax or non-existent vehicle inspections, liberal residency requirements, and no personal State income tax. But, you should look at all your financial and medical needs in determining domicile. While Escapees in Livingston, Texas has passed court muster with regards to providing mail service and domicile to RVers in compliance with Homeland Security, there are mail forwarding companies in Fl. and S.D. that are similarly patterned.
For your edification see:
Escapees TX
St. Brendan's Isle Fl.
My Dakota Address S.D.