I always heard he was just a custodian who happened to be seen by George Lucas and got the part of Chewbacca in Star Wars. However, the Master of the Majicks 3 book on Ray Harryhausen indicates that Mayhew played a live-action Trog was that ultimately cut from the film in favor of an all animated one. So what is the deal - was Peter an actor who was trying to actively find parts, or just a janitor who happened unto the Chewbacca role?
