I think you're right Amzie. Most musicians and singers do not have perfect pitch, but they are able to sing in tune. People with perfect pitch can pull a note out of thin air or name a note that is played. In other words, if I played a note on the piano, a person with perfect pitch would be able to name that note without me telling them what note it was.
I kind of doubt Elvis had perfect pitch, but he could have. As far as singing in tune, I think he did a good job of this most of the time, however there were occasions when I heard him hit a "pitchy" note. I think this happens to all singers at one time or another.