In most modern vans, the floor is the chassis
I agree Don.
However, on the 4 vans that we have had an incorporated box section that runs down each side of the van which is much stronger than the floor its self as its made from thicker steel. The diesel/fuel tank is usually fitted between these box sections, and in the case of 3 of these vans, the rear suspension was also mounted onto them.
One of the suppliers used to run a thick steel bar across the underneath of the box section and bolt the tank, not only to spreader plates through the floor but through this steel bar as well.