Yet to implement, but our staged plan is as follows:
Stage 1: Rooftop-mounted omni-directional wifi antenna compatible with our service, connected via n-connector bulkhead fitting through the roof. This is something that can be operated constantly, including while we drive. Already high because it's a bus, with a great ground plane because... it's a bus. Normally I'd rather use an nmo mount for going through the roof, but there are 2 reasons I'm going with the n bulkhead. 1: it will be easier to run larger diameter lower-loss coax to it from the inside, which I want to do to minimize loss @ the relatively high frequencies needed (plan to use LMR400), and the n-connector will make it easy to replace the rooftop antenna with a jumper cable to a mast-mounted antenna (see stage 2).
Stage 2: When we're parked, I'd like to have a mast mount on the roof of the bus, so I could get an antenna up even higher than the bus roof if needed, to clear trees / obstructions & otherwise improve line-of-sight performance. I'd run coax to any mast-mounted antenna by simply removing the aforementioned wifi antenna & screwing a length of coax between that & the mast-mounted antenna. If all I'm trying to do is get better wifi (in an omnidirectional pattern), then I just unscrew the wifi antenna from the roof, put it on the end of the mast instead, and jumper between its location on the mast to where it used to be.
Stage 3: If mast-mounted omnidirectional wifi doesn't cut it where we want to go, adding a directional wifi antenna (yagi) at the top of the mast instead of the omnidirectional to target specific repeaters.
Stage 4: If still needed after everything else, a signal booster like JD mentioned.
Here's the omnidirectional antenna we're going with. Big fan of Pulse/Larsen products, the n-male option just sweetened the pot: