My CB came with the Bus. Its a Cobra Soundtracker. My antenna is on top of the coach and its a small antenna, maybe 12 inches in height.
I believe even with the portables you need a antenna on top of you coach and not a built in antenna.
My CB only is good for reception up to maybe a half a mile. Hills, trees, bldgs, etc. all reduce its performance.
Joe C. is correct about the Antenna and the microphone.
Having said all that, I rarely use the darn thing. It's great for caravaning with other members, but its range is very limited. So, a portable s probably a good starting point if you do not want to invest a load of money for a small return.