I thought the paint looked pretty good from here as is.
Is it rusty, bubbling, or what?
FWIW, I borrowed a sprayer, and it's hands down one hundred times better than rolling. I attempted the rolling method on one area as a trial a few weeks ago, and it was not so good. Then again, you have no huge ribbed areas on your bus.
You could rent a sprayer, or buy an inexpensive wagner type. Coverage, especially in the nooks and cracks and tight spaces is sooooo much better.
Cool bus. It's pretty clean, as far as I can tell. Good find.