Adam,
One thing that you might want to consider is the added weight of the water tank. I believe that water weighs about 8# per gallon so you would be adding 800 pounds to your bus. Determine the gross vehicle weigh rating of the axle nearest to the bay that you are considering adding the water tank to and then subtract the actual weight of that axle. If the difference in the two is greater than 800# you are good to go. It would also be beneficial to check with your converter to get their opinion on the additional weight.
Tuga & Karen Gaidry
2012 Honda Pilot