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I spent some time, today, looking at various floor plans and even trying to develop new ones; and then, I read more comments from you about your intentions and that the only thing that will work for you is a break-down unit. As previously mentioned, it must be light and under 1,000 pounds.
I researched my statistics of the forum’s volunteered trailer weights, too. Here is what I found.
The lowest weights for a 5x8’ trailer ...............was 500 lbs
...................................... 5x10’ ................ 1,000 lbs
....................................... 6.5x8’ ............... 796 lbs
........................................ 6x9’ ................. 900 lbs
........................................ 6x10’ ............... 900 lbs
Among the volunteered weights for trailers, only 3 categories had enough for a good average:
The average weight of a 4x8’ trailer ...............was 804 lbs (weights of 25 trailers)
....................................... 5x8’ ................. 814 lbs (weights of 15 trailers)
....................................... 5x9’ ................. 1105 lbs (weights of 13 trailers)
The more I thought about it, the more I felt that your break-down camper might be best as a foamie. So, I looked at the weights volunteered by foamie builders.
Eaglesdare built a 5x8’ foamie and it weighs 500 lbs.
Tac422 built a 6x10’ foamie and it weighs 900 lbs.
GPW built a 5.5x13’ foamie and it weighs 1000 lbs.
I expect that your break down trailer will need extra wood and/or metal at the corner areas for bolting the sides, top, and base together; so, you may have to be even more weight frugal than the foamie builders, above. You would also need to use weather stripping at the joints, to keep water from leaking in. If you build similar to the way Eaglesdare built, you may be able to do it. I would be inclined to build something like a 5x8’ or 5x10’ “Simple” style trailer, building the cabin about 58” tall and possibly use break-down bunk cots inside:
http://www.amazon.com/Disc-O-Bed-Cam-O- ... t+bunk+bedPlease note that in one of the pictures, it shows that the bunk can also be made into a couch/sofa.
With the above manufactured bunks, you loose some much needed space where the legs rise the bottom bed up, above the cabin deck. So, alternately, you could put a mattress on the deck of the cabin for two of you and stagger a home-build bed over top with 3 legs and a chain (or two) hooked to a well braced ceiling spar, for that fourth (fifth) support that would be over one of the sleeping persons underneath. Below is a rough draft floor plan of what I am suggesting. You will notice that the upper berth is 30” wide in the torso area but tapers down to 22” wide where the feet would be. (I have slept in a bed that narrows to 22” and it is not a problem. Submarine berths are sometimes 22” wide and you've probably seen sleeping bags that taper down in the leg area.) The chain support and bed tapering gives the person that is partially underneath more room and turning comfort. The picture, below, displays a galley on the 5x10’ and no galley on the 5x8’ version. I believe that it nearly impossible to build bigger than a 5x8’ break-down trailer; but, you’ll notice that I said “nearly” .....:

With the above, you would be able to add cabinets above the bottom sleepers and put clothing tubs or whatever at their feet.
The picture below was a rough draft done for a woman with two children and I display it as yet another alternative. She was building a 4x8 trailer. In this picture, a hammock could easily replace the overhead cot.

As I have never built a foamie, I don’t feel that I can advise you further. Perhaps you can skim through the foamie section and ask for help there.
Best Wishes!