If you belong to FMCA or Escapees, they both have mail forwarding services that are efficient. Have you Post Office forward your mail to them and then you tell them where and when you want your mail forwarded to you while you travel.
We belong to the Escapees and the FMCA both have excellent mail forwarding service. We use the escapees mail service but many use the FMCA mail service. Another option that some folks use is Mailboxes are us. Personally if you are a member of Escapees or FMCA I would recommend them.
If your local post office is cooperative, they may be willing to hold your mail and then forward it when you call and give them another destination. Smaller post offices will sometimes give that level of service. Otherwise, have your mail forwarded to a forwarding service or to a friend or family member who will handle it for you. In any case, they will then have to put it in a bulk envelope and periodically mail it to you, wherever you are.
You can receive mail in care of General Delivery at most post offices. though not all of them. The Zip Code guide that every post office has will also tell which zip code (post office branch) will accept General Delivery mail. In big cities and metro areas, often only one post office will accept General Delivery. We prefer to have our mail sent to small town post offices. There is far less chance your G.D. mail will get misplaced and the serviceat the counter is usually friendly and prompt.
Try to arrange to be in the town to which your mail is forwarded when it arrives. A post office will usually hold G.D. mail for awhile, but the time varies and is up to the local Postmaste to set the policy for his post officer. Typically it is 2-4 weeks, but RVers have sometimes found it to be as short as a couple of days. You can call ahead to the post office if you are delayed or want to improve your odds. Again, small town post offices are more likely to be accomodating - and to remember if someone called! We've never had a problem except once when we had mail sent to a P.O. that was so far in the boonies it only got mail from the main post office twice a week. Our scheduled stay there fell between deliveries, so no mail! But the nice folks volunteered to forward our Hi package to our next stop when it