You need to ride the caltrain if you don't think there are tent cities there... that's all you see when you come into the city... soon as you start getting close to the final stop, tents everywhere.
There are some bum encampments in the undeveloped areas near the tunnels just south of the Caltrain station, but its not as bad as you make it out to be. That whole area around the station has undergone extensive redevelopement since PacBell Park was built.
The largest tent cities in SF I've ever seen are under the 101 Freeway at Army Street.