I do it. An outboard tank works good but is more flat than tall. I use a 5 gallon gas can. Drill out a hole for the fuel line and install a rubber grommet. Make sure its a vented tank. Insulate well against any heat source. An actual metal fuel tank is best.
I bought the first Sprinter van in California in 2004, still own it. I mounted a heavy weight marine 15 gallon outboard tank behind the right wheel well inside the van. installed a vent hose to vent it outside, the tank was securely strapped, but easy to release when needing to be refilled. it fed a Fox with 56 blower and Nissan 1500CC motor. I sold my gas tm in 2008 and installed a Bob Savage built Savage 1. I had three 100 ft cords on a reel, would pull them to plug it into the clients washing machine wall panel. also had a Yamaha onboard generator for places without power and a little giant # 3 heater... in retrospect I should have stayed with a gas TM, although the Savage was adequate and quiet for the high end homes and fabrics I routinely cleaned...Imagine having one client that owns 144 Hotels around the world, or another that runs a major museum and has a couple of $85,000 couches... Gary Heacock was the fellow that mentored me, rest his soul...
A stroke last year that briefly paralyzed my left side helped me identify what is really important. God and family.