Dont assume the bus sits level in its normal operation position and weight. Mine does not. The rear suspension sits the rear higher than the front so if i were to put a black water tank behind the the rear wheels and mount it flush I would have to mount the evacuation valve towards the front to get optimum drainage.
I am building a deck on top of mine. A deck that I want to be reasonably level as well as match the lines of the bus. My bus is parked on a significant compound grade change, ie water would flow to the rear left corner. I am using the lines of the bus and and what I call a "red neck" level to install it. My leveler consist of some magnets, a 10-12ft piece of angle iron and a tape measure. All reference points are made from the bus itself. Out of square maybe but it is equally flat and a the bottom landing and stair support were built to "match up" in this fashion.
This may all change as I add weight. The important part is the buses lines match with the additions, no matter what.
All that being said I would luv to have four of those jacks.
btw...I know how to drive firetrucks too...