2 types of propagation with frequencies in that range:
1) line of sight: What most people want for communications while traveling. It's exactly how it sounds. The higher your antenna the longer the range to the radio horizon (which is just a bit further than the horizon itself). Getting height isn't a problem on a bus... mount it centered width-wise on the roof. But then you have to worry about ripping it off under overpasses. So sometimes a front cowl or window mount is a necessary compromise (can also use fold-down mounts assuming you remember to fold it down when needed).
2) skip (off the ionosphere) propagation. More a factor at night, and usually detrimental to communications. But if you want a CB more for entertainment that communications, then skip could be a good thing. Don't really care about height with skip, just a clear path surrounding the antenna. The thing about skip is, you'll hear stations potentially hundreds of miles away, but will be deaf to stations between your line-of-sight range and the skip zone. So it's kind of sucky for comms in general.
The thing you do NOT want to do is mount the antenna behind the bus. Like all the jeeps you see with the antenna behind their spare tire on the back? Pretty much the worst thing you can do.
If you want to minimize skip and maximize line of sight capability, a 5/8 wave antenna is a good choice. It flattens our the radiation pattern, directing more energy to the horizon and less towards the sky. But 5/8 wave at CB frequencies is LOOONG. They do make those firestick-style antennas in 5/8 wave (I think, not really big on CB radio, so I could be wrong), but you don't get a compact antenna without compromise, and they will not perform anything like a straight 5/8 whip would.
