It's not easy to mount a CB antenna on most class C's and have it work reasonably well because the structure doesn't give a good ground plane. I've thought about how to do it from time to time and generally come to the conclusion that I would do without a CB radio instead.
The one time I really would have liked to have one was in Alaska when driving the Dalton Highway; there are a couple spots there where it's very wise to announce one's presence in case someone (particularly a large truck) is coming the other way; there are even road signs stating as much.