U.S Postal Service: Whether your plan is to take your RV on the road for a lifetime, or just a weekend, the U.S. Postal Service offers a few different options.
If you plan to stay long-term at a particular location, you may be able to install a mailbox so your mail can be delivered right to your RV lot. But it is also possible to have your mail delivered at the entrance if your campground permits it.
RV travelers can also rent a post office box at a nearby U.S Post Office. This option is great if you plan on traveling past that location frequently to pick up your mail. If you are considering this option, you should inform friends, family and companies you expect to receive mail from that you are temporarily changing your address.
If you don’t want to change your address and rent a post office box, another option is placing your mail on hold. Mail can be held at your local post office for up to 30 days.
Forwarding Mail Service: Whether you’re a full-time RVer, or you travel for weeks at a time, you can hire a private service to forward your mail to you while you’re on the road. Most of these services have the same process. First, you change your address to one the forwarding mail service provides. The service then sorts your mail per your request and can digitally scan your mail and email it to you, open your mail upon your approval, and even overnight your mail based your RV’s location. Communication is vital for these companies for security reasons, because they will not open your mail without your consent.
Source
https://blog.allstate.com/getting-mail-while-living-on-an-rv/