There are several mail services available for full time RVers. Escapees has mail service in TX, SD and FL. St. Brendans Isle in FL. Traveling Mailbox has addresses in many states. We went with the Passport America service (which I think is also the Good Sam service as well). Our mailing address is in FL. We chose that one based on the cost of health insurance at that particular zip code.
Whichever service you choose, the basics are the same. You have your mail sent to the mail service address, and they will forward it to wherever you are. You pay the postage to do that. They will also shred and trash any selected mail items you direct them to do so. You may also have selected mail items scanned so you can look at them online.
The costs vary between services, but overall, not greatly different.