I've looked at a lot of different utility bed, flat bed, and trailer/hauler bed mfg pages and not really found what I was looking for either. A utility bed could work, but it would need to be one of the relatively rare low bedside versions to not force the camper to sit ungainly tall on a spacer in order to clear the more common 'deep' bedside tool boxes.
I have played around with modeling this. I think that you could actually sit the camper lower on a custom built bed (assuming cab roof clearance) than you can get in a regular pick-up bed as there are supports under it that add height. Start with the camper's actual foot-print and build a frame just for that. Attach it to the truck's frame. Add support stringers to your frame between the rails of the truck frame. I don't think that you even need a floor in that frame, just some thin stock to keep the camper's wood bottom from being exposed to the road debris/grime/weather-induced yuckiness.
Then add the storage compartments as desired & needed. Keep in mind your fuel filler neck(s) and what you might need to do if they need to be relocated.
I don't see a winner in aluminum for several reasons. You need to electrically insulate it where it comes into contact with steel or moisture in the gaps will create a corroding battery. It's weight is 1/3 or so of steel, but so is it's strength. Unless you have a structural or aircraft design and engineering background its going to cost 2x-3x as much as steel and end up weighing about the same.
Your construction plan sounds pretty similar to what I was thinking- start with the footprint, move out to the wheel well/fenders and then construct boxes, bumper, and side rails in the space that's left..
Do you have a lot of fabrication experience? I know there are issues with aluminum, but I've really enjoyed the bit of time I've worked with it and I'm intrigued by the possibilities that come with doing it right..
