For generator, rather than Onan, you might want to look into a Power Technology generator. They are far better built, and they are more reliable.
PT-12KSiC | Power Technology Southeast Inc
I'm not sure I would go with one of those little yamaha inverter type units. Especially for powering rooftop air conditioning. The only downside is you are paying for the reliability and power quality of the Power Tech generator. The last quote I got for the 12kw unit (with 240volts) was $10,550 new.
Still, it sounds like you are trying to make a business out of this and going down because of lack of power isn't something you can afford, I figure.
As for the split units not holding up on a bus, I have yet to see any reports of this. Mind, I have yet to see many installs using mini-splits, either. It is what I plan to use on my bus, but that project has been delayed by life and other projects. Ah well. As always, good luck!