I think the tank design will be one of the major features of the gardiner systems if I remember what alex was saying right. He was talking about tanks custom made for each van, fully baffled (Unlike the tanks available off the shelf) and covering the entire floor of the van. Because they will be 4 times as wide they only need to be 1/4 the height. I assume there would be some mechanism to keep the water flowing even when you're parked on a hill (Simple enough to do really).
I guess that a tank covering the entire base of the van could be fully bonded to the floor and would be very easy to make crash proof too.