the USPS may have different criteria about extending routes with non-permanent homes.
also if a person puts in a mailbox for a non-permanent homes, even though they might have stopped or forwarded the first class mail, it will still get things for ‘addressee’ and ‘occupant’ year round. it is their responsibility to remove that from the box, if you create a problem then the USPS can stop all delivery to that address.
Rural Free Delivery, AFAIK, was never to the farmhouse door, but to the nearest public road. And maybe not even that, if there was a cluster of mailboxes at the end of a route.
As far as getting an address (around here, we call them “fire numbers”), just think of how good an address like mine once was if you have an emergency?
It was “Musicat, Rural Route 3.” RR3 was about 30 miles long, and not in a straight line, more like a gerrymandering drunk. Hey, it worked at the time because everyone knew everyone and we had party lines if we had phones at all. No secrets, no confusion, but that was Then and it is Now.
That hasn’t been true for many years, here in Minnesota.
As of about 1991 or so, your address would have been “Musicat, Rural Route 3, Box 123.”
That hasn’t been true for years in Wisconsin, either. My example was from long ago. Long ago, but not forgotten.
At least that would narrow it down to a specific address. Without the Box number, a fire truck driver would have to know where Musicat lived on RR3 (or look for the smoke).
My rural relatives already had box numbers going back as far as I can remember. The big change around then was going to house numbers. Really, really high house numbers.
the RR as mentioned could be long circuitous and have one route contain portions of a number of public roads. good enough for the post office. with hundreds of boxes you would need a box number or name on each one.
fire numbers existed separately in a linear but not necessarily contiguous fashion at the end of each driveway and not necessarily near mailboxes.