Motor homes are strange critters. The floors are nearly always plywood on top of the unibody chassis built behind the van front subframe. Soft floors usually means the plywood is junk. The condition of the unibody frame boxes under the floor will make a difference as to what you can put on top of it.

The "inverted top hat channel" originally had a top steel flat surface. The odds are about 50/50 that that top steel surface is intact or not, and how much junk has fallen inside the box frame. Those boxes tend to rust out from the inside out. If the top steel surface has holes (or sections missing) you need to remove the top flat surface and dig the debris out of the frame box and assess its condition, rebuild the box as required, and add a new top steel surface to the frame box.

If the inverted top hat box is in good (or repairable condition), adding a new floor to the motor home chassis will make it as good as it ever was, and you can use it for any purpose up to the original weight capacity. Using a strictly flat bed will allow the chassis to have some flex, adding sides and a roof to the new floor will stiffen up the chassis a lot. Gene