Yes, I do....
I mounted my 15 gal barrel transversely behind the fuel tank. The drain line drops through the floor on the driver's side behind the wheel well then goes back around the bumper bracket before turning back toward the tank. With the frame rail in the way that was the only way to get enough drop on the drain line. Damn those frame rails!
I screwed the plywood box to the wood bus floor then used aluminum bar stock and all thread rods to cinch the tank up into the box. Used auto undercoat spray to waterproof the box before mounting it.
Here's the drain line coming in to the tank. The clear plastic tube is the tank vent.
There was room for the barrel between the frame rails and the body trim panels but if I mounted it there the shower drain wouldn't fit. Plumbing in tanks is a bugger in the short buses.