If you run into space problems there are smaller tanks than the typical 5 gallon (?) bbq tanks available. I was in Sportsmans Warehouse the other day and saw they carried ~3 gal tanks and ~2 gal tanks whose diameter is less than the fatboy bbq tanks. They were priced at ~$60 each which is way too expensive tho'. I use two of the 3 gal tanks which only cost ~$33 at a local propane supply shop.
In a motorhome I refurbished a few years ago I had a sheet metal/HVAC shop make me a cheapo metal box (with one face missing) for the propane tank. I installed an exterior hatch from ebay and added a small exterior vent and sealed everything with caulk. I think this is the way things are supposed to be done. I originally considered an exterior mount but I recall seeing a thread somewhere that exterior tanks are now illegal in places.
You might want to check with FWC or Ben at ATC and see about the possibility of the furnace cabinet option without the furnace if all you need is a tank and cabinet.
My rig is a Falcon shell (doghouse) which is shorter than the pickup bed. The space between the camper and tailgate is perfect for 2 propane tanks, a genset, and gas can.
Rob