Prof:
I use slotted angle iron (avail home depot, lowes) fastened at the base of the walls and bolted to the floor wood. I mark, on the floor wood, the preferred location of the fasteners, then I remove all the wood from the van, drill 1/2 inch hole where marked on the floor and on the under side of the floor I insert a "T-nut" (home depot). Now reinstall everything and fasten the walls to the floor using using 3/4" long machine screws or bolts thru the slotted angle and into the T-nuts. I dont fasten the wood directly to the floor beecause I cut it to fit the floor profile; it won't budge. If you prefer, you can screw your floor wood into the floor using all purpose dry wall screws or self tapping screws ( no drilling required for either) Fasten the rear ends of the wall to the door frame and you need not fasten to the wall ribs if you've got a forward wall fastened to and running perpendicular to the side walls.
this all sounds clear to me as I type, I hope you feel the same.
Buy wood soon as the price is increasing daily; it's gettin quite expensive due to the rebuillding of afganistan, Iraq and now the hurricane damaged southeast.