Not the OP, but I've been using St. Brendan's Isle[1] for about a year and it's been great. I've also heard good things about My RV Mail[2]. Both are in Florida, I think mostly because it's a popular state for full-time RVers to establish as their domicile -- no income tax, low vehicle licensing fees, etc. I looked at bigger name services like Anytime Mailbox and Earthclass Mail, but found a lot of complaints about reliability, hidden fees, etc. in the reviews. Many of these services contract with local businesses (like convenience stores and independent shipping shops) to actually receive your mail, so your experience could be great or terrible depending on the address you choose. St. Brendan's Isle and My RV Mail both handle all mail in-house.

[1] https://www.sbimailservice.com/ [2] https://myrvmail.com/