Re: water tank above head height
The weight being that far back should not be a problem. Look at the data plate for the bus, and the front axle/rear axle weights. The rear axle with duallies should be rated to support a lot more weight than the front. Putting the weight behind the rear axle tends to take weight off of the steer axle, so steering would be lighter, but resistance to cross-winds might go down some as a result.
If you really want to get picky about it, calculate center of gravity like an airplane pilot. Pick a "station" to measure distance from (firewall, front axle, rear axle, front bumper, etc.) and calculate the desired center of gravity for a full load. I haven't done it with a bus, but with some head scratching it should be possible. Then have the bus weighed empty by axle or by wheel, and calculate the actual center of gravity you are starting with. The difference between the empty and the maximum would tell you where the load weight should be placed.
Or else don't sweat it. As far as being top-heavy, as long as you drive cautiously like Grandma, and aren't trying to take corners like Mario Andretti (Danica Patrick?), there should not be a problem. I think some skoolies have transported water barrels on roof decks.
You are already aware that a "smooth bore" tank will have some sloshing and delayed weight shifts as compared to a baffled tank. As long as you plan your stops and corners a little bit, you should be fine. Smooth bore milk tankers roll down the interstates every day.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.