Here's an interesting wrinkle to an interesting problem. We have used Escapees mail service for three years without issue.
However, sometimes for prescriptions, that's not a solid solution. So we use the local RV park (most public parks don't allow this, but commercial ones typically do).
The wrinkle came that we could not predict which park we would be in when the drugs came, so we used General Delivery, also an option most everywhere. The wrinkle was, we left town before the drugs got there. Then we used Mail Forwarding in the new location and General Delivery there, to move the General Delivery from the other location.
Complicated, yes. But when you don't want to backtrack a hundred miles for a piece of mail, it's still a solution.
Moral of this story - you have to think about your problem before you deliver the solution - there's always more than one way to skin a cat. 8)
The Postal Service can work, but you have to understand some stuff.... ;D