It was a bit before then, in the mid-to-late 80s, when I was doing a fair amount of geology and geophysics work in the Las Vegas area. You’re right that food was cheap back when the casino management thought they had to have inexpensive restaurants to get people to come and gamble. Eventually they discovered that people were going to come lose their money anyway, so prices went up a lot.
It was nice back then, though, to hit the Flamingo buffet after a long day in the field, and stuff yourself with good food for about $10.
Since I had to leave at 3 AM to get to the tunnels for the morning shift, the breakfast at that hour was so cheap, the tip was about 200% and still just a couple bucks.