Assuming you live in Alaska, as per your avatar, then I can see why you want to be able to sleep in the vehicle.
Before I make any comments, I would need to know what vehicle and interior configuration you have and how many people you want to be able to sleep (I'd otherwise assume two).
You would need to consider what materials to make it from - steel frames and side skins with aluminium tops would be the strongest and possibly lighter than the thick ply alternatives, but would also tend to sap heat away from your bedding and is more expensive and difficult to build.
You also need to let us know whether the vehicle will be a dedicated camper or will be a multi-purpose vehicle that has to allow rapid removal of the system or easy reconfiguration for carriage of other passengers or goods.
I'm looking forward to this discussion and your project, because it's an interesting puzzle. i have several ideas on how I'd do my 109 for different criteria, but I'm limited by it having to fulfil many roles.