RC Cola's diet version was Diet Rite. According to http://www.dietritecola.com Diet Rite was the very first diet cola, introduced in 1958. Nowadays Diet RC and Diet Rite brands coexist.
My parents used to drink this before the introduction of Diet Coke in the 1980s. They tried their hardest to get me to like it but it was absolutely vile. Hardly any sweetness and an aftertaste that hung in your mouth forever. What's the fun of soda is it isn't sweet? Might as well drink straight coffee or tea, and they taste much better.