Sportsmobile tanks are likely small because they would have to charge a hefty sum in order to have them custom rotocast like RT's, or would have to have to sell in volume like Winnebago. Since they are a custom shop, one-off-ing tanks would be expensive.
It just might be time to go visit their shop again... I know they can do some cool stuff with the high roof Transit. What I'm curious about is the high roof Transit coupled with a Danhard A/C unit. This means no roof mounted A/C, so overall the van may be lower in height than a standard "B" with the A/C unit on top.
Of course, the EB engine will make the hills a nice drive, due to less power loss at altitude.