Electric queen bed platform construction (removing bench)

We have an '09 WW FB2100 that has the Happijac electric lift queen bed system in the back. The lower bed is a fold up couch which we would like to replace with a second queen bed. Removing the couch looks simple and the couch lift rails could still be used for a mattress frame box. We can make the bed frame box. The question is does anyone know how the factory box is built? The perimeter frame is just pieces of probably hardwood. The platform is hollow and seems like it might be a top/bottom sheet of plywood over 2x3's or 2x4's lying flat. How thick is the plywood and what is the spacing & direction of the 2x3' or 2x4's ?