LSretired1 - Do you have a rear box on an Aluminess bumper? If so, the back side of the box provides a great location for mounting a 10L / 2.6gal tank. With a burn rate of .03 gals/hr for a diesel heater you will have around 70 to 80 HOURS of heater running time. This is an easy install.
A harder install would be under floor do to locating the filler in a convenient place. I have thought about the space outboard of the side door step with an filler access port through the interior floor.
Same concept could be used in the rear area of the van to access a filler. The extended vans have an often ignored great space outboard of the frame and behind the rear wheels. I used this space to install a 5.5 gal water holding tank on each side for 11 gals of gray water storage. You could locate a heater fuel tank there with a weather tight access port just above provided your interior layout will accommodate that.
A strong consideration is how often will it be necessary to fill the tank w 80 hrs of actual running time per 2.5 gals usage. Also, a nice feature of the aux diesel tank dedicated to the heater is the ability to use kerosene which burns cleaner.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com