I got the Sportsmobile tank, which appears to be custom made to fit on the driver's side of a Ford van. The mounting ears are designed to hang the tank from the floor with through-bolts. The rock guard is sized to fit in the space.
the valves are on the side and designed to work with the SMB access door.
I like it because the tank is tucked up out of harm's way, and the valves are accessible. We also have our electrical inlet under the cover.
The tank, regulator, and cover are available from Sportsmobile West and the prices are competitive. We got the bigger (10 gallon?) tank and it is overkill. But the smaller tank is the same diameter so it takes up about the same space.
If you have rear air conditioning I don't think the tank can be mounted here because the AC piping goes through the same area.