When my parents were 'homeless', (aka full-time RVers), we did the General Delivery package thing - mostly vehicle registrations, tax forms, and so forth.
Another option would be to mail stuff to the campground you intend to use when staying in the area. That way you don't have to worry about the PO being closed for lunch, for the weekend, or holiday. We did that with the folks a couple of times as well. Of course that only works in a staffed campground... Many of the USFS campgrounds have no on-site office or staff.