On the RT homepage the FAQ has a description of how they adopted the whole King Arthur theme:
"What is the meaning of the "shields" in the Round Table Pizza logo?
Round Table Pizza was founded by Bill Larson in 1959. The first restaurant was located in Menlo Park, California. Mr. Larson wanted to have a fun, casual pizza restaurant for his friends and family to enjoy. Because he was focusing on his own family and friends, he insisted that the pizza had to be the best. He was the person who started the policy that every Round Table pizza's ingredients would be fresh and plentiful, and that we would roll our dough fresh every day, in every restaurant. And after 40 years in business, we still do. As far as the shields go, the original concept of "Round Table" was to signify a gathering place - it had nothing to do with King Arthur or the Knights of the Round Table. One day, in about 1961, a friend of Mr. Larsons was eating pizza in the restaurant and started drawing the characters from King Arthur's court, all eating pizza. Mr. Larson was so excited he adopted the King Arthur Theme and started making the restaurants look like English castles.The shields were added to the logo (the name) about 1970. There are actually three shields and believe it or not, they symbolize the letters "F", "U", "N", spelling FUN! Most people don't see that when they look at the shields, but that's the true story!"
I don't recall many commercials that played on that theme, then again it's probably before my time. I remember most of the commercials in the 80's which had the funny looking guys like the following:
I love it. I guess people who eat RT are either fat or dorky. LOL. They certainly had some great marketing.