I painted my full bus in the same way that skoontz did. One can of Valspar enamel hardener to a gallon of basic Rustoleum paint. To get the colour I wanted I mixed 3 parts John Deere green to 1 part grey (or off white?).
Don't mix too much at once. The stuff will start to set up in 15-20 minutes or so and the paint will become thicker and stickier from then onward.
I rolled as much of the paint on as possible using a short nap roller. Using the "roll and tip
" method helped to smooth out bubbles and spread out some of the runs.
Foam brushes took care of the finicky areas. In my experience, even the foam brushed areas could be smoothed out so brush strokes weren't too obvious by "tipping" the paint after putting it on thick with the brush.
Doing the roof you have the benefit of gravity being on your side! Runs should be at a minimal.