That's the safest place to put a tank... ya know... if it was underneath
where spare tires are now... any rear end collision would cause a rupture
and possibly explosion dut to ignition from hot engine or spark.
Where Mercedes put it... a rear end collision will not damage the tank..
that's the safety crumple zone... If it is hit from side, it still take a
lot of force to get past the wheel, wheelwell, and the strongest part of the
frame that holds the roof up.