The song started out as a satirical comment on immigration, pakistanis and the racists of the day. The main lyrics of course changed, leaving only the chorus, "Get Back to where you once belonged" - bootlegs exist of the track which contain original lyrics - every few years or so these are "un-earthed" and Paul is forced to explain the apparent racist content of the song.
The song evolved in the studio and the finished lyrics don't particularly mean anything - although Linda Eastman did live in Tucson Arizona for a short while! The stories of Jojo and Loretta were most likely written between Paul and John.