I use a standard 20# propane tank, but I mounted mine at the rear of the bus just behind the "nut" (differential). The tank sits on a 1.25" thick (two 5/8" thicknesses) platform of plywood that is suspended by 3/8" allthread.
After the platform was constructed, it was closed in with 3/4" plywood. It is open on top, and there are a few 1/2" holes drilled in the bottom for drainage of gas or water (propane is heavier than air, so it needs to be vented downward).
(Crappy picture, sorry.)
The platform is just barely wide enough to fit two 20# tanks side by side. I should have made it a tiny bit wider for convenience.
I figured that no road trash would come up through the nut. You can barely see the tank mount from outside the bus, and I don't think that I lost any practical amount of ground clearance.