On our 2005 Westfalia Airstream - propane location is attached to the chassis, right below the (passenger side) sliding door, and features a protective, latching faceplate to cover the knobs and valves.
If I were adding a propane bottle to a converted Sprinter, I'd consider a custom mount on the rear door similar to the spare tire mount systems available. I may eventually eliminate the under-chassis location just to provide added clearance for those rough driveways and pullouts where the road bed drops off sharply or the approach angle gets a bit dicey.