We had one mounted like that on an E-150 2wd. The guard got smashed up a bit and we did no off-road, only rough roads. On a 4x4 van there is more ground clearance so it might be OK.
The tanks are different for the 2 mounting positions. The lower mount one has valves that point downward at a 45 degree angle, wherease the higher mount one with the access door has valves that point horizontally. There are 2 tank sizes available that will fit in that location. 10" x 23" tank is 6.9 gallon, holds 5.5 gallons of propane. 10" x 32" tank is 9.9 gallon, holds 7.9 gallons of propane. If you want the higher mounted one Sportsmobile West sells both the tank and the access door for a reasonable price.
Here is the larger tank mounted on my van (need to get some better pictures), and the parts.
We wanted the bigger tank mainly for winter camping and skiing where we sometimes spend 2 or 3 nights up in the woods away from propane dispensers, with the heater running all the time to keep things from freezing. The smaller tank should last several days in milder weather.