Met an Austrian guy who biked from NYC to LA in the early '90s. He had a paper list of people across the country who were bike tourer friendly who could house him, and he'd call them on payphones. He didn't have a tent, so he'd also sleep in post offices.
I never slept in a post office, but rural firefighters were always very good to me on bikepacking trips. Plying me with food and letting me sleep in their gym or somewhere around the station.