...One could level out the floor and put cut outs for batteries, use flat rectangular tanks between the two floors. I think some people who are in the 5 foot range could do the same with a HR now.
That has worked out well for me, although I didn't use the "basement" for tanks, it would be easy to do.
In my case I wanted the extra storage instead of more air space (head room) at the top of the van. At 5'5", we still have plenty of headroom and the storage is usable for various purposes. Originally I wanted a space to carry 4x8 plywood sheets or 2x4s under the floor, and I still use it for that. But I have added drawers so I now keep all kinds of stuff in the basement.
Back storage = all my shore power hookups, hoses, telescoping ladder, floor mats for van entry, axe, knife, saw, butane stoves, frypans, cooking utensils... and a portable folding 4' x 6' model railroad layout!
Side storage = 2 HF outdoor folding chairs, extra shoes, trains & cars (see above)
I ended up with about 68" above the finished floor to the almost finished ceiling... not sure where I would have put all the stuff in the basement if it wasn't there!
Added benefits: 1) step up into van is still less than a transit, 2) floor is flush with cab floor so walkout from cab to back area is level. Basement is 6" high