"7 3/4: PART TWO"
In the early 1960s, Ralph Malph received an honorable discharge from the army and return home to Milwaukee. He decided to pursue a career as an optometrist like his father, Mickey Malph, and so he soon departed for college.
Ralph Malph was never again seen in the TV series 'Happy Days".
Here is the Toobworld conjecture:
While at college, Ralph discovered a new passion that was more in keeping with his nature. He became a student of filmmaking, and years later got a job in Hollywood as a film editor.
The Hollywood life changed him. As Ralph got older he put on some weight, as can be seen in these pictures in which he is being interviewed by Donald Hollinger of NEWSTIME Magazine.
The recastaway problem gets a free pass from being a Zonk when the splainin involves the aging process. I don't know who played this film editor in the 'That Girl' episode, but I see no reason not to think that it was Ralph Malph.