Ralph,
I was going to wait till my bed conversion is done, but I'll post some progress photos now. It's a bit like seeing sausage being made at this point.
My goals are a bit different from Cheeze's. While keeping a conventional height on the bed, I wanted the maximum cargo opening. I'm not hinging the top, because the storage under the bed is more of an outside storage space to me. My bed top will be removable but it will be more like the tops of the dinettes.
I am not a great carpenter, so I wanted something quite simple to build. Mine will be about on par with RV factory standards.
Due to my goals of maximum cargo door opening, I decided to use an angle iron to carry the bed along the outer wall. And since the cargo door protrudes beyond the wall by 1/8" or so, I decided to make a shim using paneling, to shim the wall outward in the affected area. I made a management decision to cannibalize the bump out thing that I had removed from the wall. I needed the paneling it had. So the first photo shows the shim pieces. Three pieces because the paneling was not as wide as the bed area.

This next photo shows the angle iron installed over the shimmed wall. It is well-supported by the four upright wood pieces in addition to the wall screws. It is stout enough to span the 40" door without deflecting any real amount when I put weight on it in the middle. This angle iron is some stuff I had from work.

Then I took another panel and laid it over the shim pieces and right down onto the angle iron. This is to provide a barrier between the bed and the cargo door. Both visually and as a draft barrier. The panel was again a couple inches too narrow so I left the gap on the fridge end. I will try to figure a way to scribe a piece to fit as close as possible, but the ultimate plan is to cover the whole thing with some sort of padded or quilted cover.

OK, that takes care of the rough construction on the outer wall. The cargo door has a full 12" high opening, which is pretty useful to me. Now I used another piece of the steel on the fridge wall. The reason I used it again was that it was able to be screwed into a spot where there is structural wood. Everything lower than that is just paneling or some light structure that is basically just hanging there. The wheel well doesn't support any weight. So here is that piece.

This was today's progress. The next step is to build a conventional RV type wood box for the other two sides, and also support and cover the end where the steel sticks out past the fridge. I'll be using 1/4" luan for the paneling, and possibly some scraps of 1/2" birch plywood for a trim rail around the upper edge. Most of the framing will be 1x2's. The top rail will be two 1x2's with the luan sandwiched in between. The rail will be supported by 1x2 posts every foot or so, going down to 1x2 base plates screwed through the carpet to the floor. Just about like the dinette.
The top will be 3/8" plywood, I think, and will be have some bracing underneath it. I have to experiment a little with that part when I get to it. I will post more photos as I can. This weekend is supposed to be very rainy and I don't want to haul the plywood from the big box store in my truck if it is raining, so I might not get anything done till next week.
It looks pretty rough right now but eventually it will be decent, I think.