I'm planning on having two solar water panels, probably home-made, to complement my eight PV panels already on the roof. I'll keep them simple, maybe using a long coil of black-painted copper pipe inside a shallow box with a glass cover - home-made evacuated tubes made from old fluorescent lamps are beyond my abilities! My biggest potential problem may be to prevent the water inside from boiling and flashing into steam on a hot day, so some sort of automatic drain-back design based on water temperature will be necessary.
With 2kW of solar I should be able to also heat my domestic hot water in the afternoons, after the batteries are fully charged, using excess power from the panels that would otherwise go to waste. If you want to fully carpet your entire roof you could have a lot more PV than me - my eight grid-tie panels occupy about 22 feet of roof length, leaving about 7 more feet for water panels. One incidental benefit to covering the roof is that the bus gets much less hot inside on a hot day.
If you're interested in heating your glycol in wintertime you should check insolation data for your chosen winter locations: http://rredc.nrel.gov/solar/pubs/redbook/
The amount of solar watts per square meter and the number of usable solar hours per day will determine how much panels you'll need for any given amount of BTUs heating. The good folk at the Northern Arizona Wind & Sun forum have all the answers I ever need for matters PV, so it may be worth your while to peruse their cornucopia of wisdom and experience.