When we were making a long move and temporarily without a stable address I looked into getting a PO Box and it seemed impossible without a real address.

I ended up finding some kinda sketchy-feeling services aimed at people RV living, and not much else. I wasn’t able to find an official solution to the problem of “I need to receive mail but have no address” (there may be one, but in solid 60-90 minutes of searching I didn’t find it, but did find a lot of people complaining about the problem)