[aerohead]

That's the general idea.
The base would be flat against the trailer 'floor.'
With an inelastic fabric, a tailored shape would hold true to it's pattern, even without interior 'formers' and 'stringers.' ( I've seen a scale recreation of the Space Shuttle, including the two external Boosters, which had high fidelity to the real thing in spite of no interior 'supports.'
These aerospace, coated fabrics, used on inflated outdoor advertising are perfect for the job. Rip-stop fabrics are much lighter, although often lack UV radiation protection ( which ruins Nylon in short order ).
Once inflated, air is no longer moving, and very little blower power is required to maintain the necessary static pressure.
A reversing valve evacuates the air.
A zipper opens the fabric for removal of the other tarps, if stored inside.
The whole thing rolls up and stores until needed, and could fit into a dedicated, recessed alcove, built into the bulkhead. Cargo could then be placed directly against the bulkhead.
The forward gap-filler ought to be always be used.
Same for the two at the rear.