You can sign up for mail forwarding with several companies. I know of three that do this specifically for rv'ers. We use alternative resources in sioux falls, SD. They give us an address where we have our mail sent. They forward to where we are on our instructions. We have the weekly mail plan. Another is AmericanHomeBase in Pensacola, FL. And Escapees in Livingston, TX. Some folks use places like Mail Boxes, etc.
I think the urls are as follows.
www.alternativeresources.net
www.escapees.com
www.americanhomebase.com (not sure on this one. might do a search)