Of Muppets & Men: The Making of The Muppet Show (1982)
https://www.youtube.com/watch?v=eaBmbh3aWk0 - from the mdftrasher YouTube channel (59 minutes long).

The blurb:
“Documentary about the work of puppeteer Jim Henson and his team in creating The Muppet Show.”

Fun (random) fact; Roger Deakins was the camera operator on this documentary.

60 Minutes: Backstage at The Muppet Show (1979)
https://www.youtube.com/watch?v=3X9DVE2J-ho - from the PMC YouTube channel (14:30 mins long).

The blurb:
“Behind the Scenes at the Muppet Show from a 1979 Episode of 60 Minutes.”