Wow. I looked at that link. Nice job, but overkill for my purposes.
I sleep in my bed on occasion.
All I did was put a 2x6 between the indentations, then put 2x4 legs on a sheet of plywood that straddles the wheelwells. 3 pieces of plywood makes the platform. The front and rear plywood sets atop the 2x6's and pieces of 2x6 screwed together into an angle. These are removeable supports for the front and rear plywood sheets for stability.
So, the whole project (made from scraps) is 2 flat plywood sheets, one plywood sheet with 4 legs screwed to it, 2 lengths of 2x6 and 4 angle legs. It all stands on end in a corner of the garage when not in use. I throw my futon mattress on top and sleep just below bedrail height, so I can see out the topper windows and feel the breeze coming thru the screens.
I used to use an air mattress, but the futon matress is more comfortable, warmer and rolls up for storage (but takes up a lot of space). The futon mattress is on the porch swing hanging in my garage, like a couch for guests.
It cost absolutely nothing. All the wood is salvaged scrap and the futon matress was the thin one that I replaced with a 10" thick one. I kept it knowing it would be great to camp with, even if it was in a tent. It is bulky tho', compared to an air mattress.