I thought that I would be smart (not!) this winter and try to open a PO Box at the USPS post office here where we are staying for the winter. The USPS wanted a house deed to open a PO box for us (who carries a house deed with them on the road??). Apparently there were work arounds available but the unhelpful lady at the desk didn't suggest any.
I gave up, our house checker collects our mail, worst case is that she will mail any important looking (brown envelope) mail to our daughter who will scan and e-mail it to us.
It's interesting that the public mail services in the US and Canada don't help themselves to at least survive in these days of declining usage.