Big thanks to everyone who has posted ideas into this thread! I've taken lots of inspiration.
Here is my system (version1, always leave room for improvement). I have a conversion-van bench seat that folds back into a short bed. In order to extend the bed to a full 72" long, I wanted to incorporate a bed platform into the storage mechanism and be sure to waste NO precious space.
The system starts with a box that snugs up against the back of the bench. It's 3/4" CDX plywood, screwed and glued. Inside covered in brush-on bedliner, outside carpeted in a decent match to the OEM carpet. One side of the box holds the house battery and electronics, the rest is tool storage for now, but I sized the compartments for an Espar D2 heater if I can find one at a reasonable price.
The tall rearward edge of the box also supports the forward edge of the bed platform. The platform attaches here with removable pin hinges so that I can pivot the platform up to access the jack compartment on the right side, and things underneath.
The support legs attach with knob-headed bolts so that I have a tool-less way to remove the legs and break everything down so it can store flat in my garage when not in use. There are tee-nuts in the platform for the legs, and additional nuts on the corners so I can add longer (telescoping) legs and use the bed platform as a camp-table if needed.
Storage underneath the platform is based around bins for easy packing/unpacking of the vehicle (also my daily driver), so I can quickly move the bins from van to shelves in the garage, and back. The storage area will take three large Roughneck totes, or two large plus one "short" bin with my Coleman stove on top, or two bins and a 7-gallon water jug.
There's room under the bench/bed for a water tank, so eventually water will be plumbed in a bit more permanently, but for my solo shakedown trip to Overland Expo last month, this was more than enough room for all of my gear.
More details and photos available
in my build thread, and even MORE pictures available
in my web album.