We've used the Escapees RV Club for 18 years for our mail forwarding and they are great. They currently have a membership deal through October as part of their 35th year in business.
Full-timers mainly, but not solely, are domiciled in Texas, Florida or South Dakota. If you need to purchase medical insurance South Dakota has some problems on that. You need a good PPO plan to travel and full-time.
There is no one state to domicile that fits everyone. You need to do research on your tax issues, medical, registrations, etc. What may be good for one won't necessarily be good for you.