Re: weight limit in penthouse
What year is your rig? There might be differences. My 1997 had a one-piece upper bunk. I typically slept on the couch, but when I had a friend traveling with me for six months or so, he slept on the upper bunk. Fairly big guy, but only one person (weight around 250#). When he was up on the bunk it had a definite "U" bend in the middle. Eventually, the footman loops that were in the bunk to hold it up to the ceiling came out, and the fabric covering the bunk tore. Um okay, guess we had better fix that!
We took the bunk out onto a picnic table and removed the bottom panel (which is the "ceiling" when you are looking up at it from below) and then the fabric. Then we could see the whole thing and how it was made. It was a full sized slab of plywood (3/4" IIRC), with a perimeter strip of the same ply that was around 3" wide. This was screwed [sorry, make that STAPLED, not screwed] to the other piece (no glue or etc.). The reason the footman loop came out and the fabric tore was that the strip and the slab were separating (hence providing no strength) due to the staples just pulling out vertically.
Had I been at home I would probably have made a whole new upper bunk - it would be easy to make it much lighter and much stronger. However we were out road tripping. So what we did was remove the entire perimeter strip and reinstall it with wood glue, plus screws. Then we put it all back together and let it set up. After that no more sagging, the footman loops were put back, and I sewed the fabric back together. Did fine for the rest of the trip, and no more "taco" sleeping.
So, it seems like they are not all made the same. Mine had never been used by the original owner (they literally had it in storage the whole time they owned the van), and started failing right away with 250# just lying/sleeping on it. However, it could have been both stronger and lighter with a better construction method. The fabric and "ceiling" panel could be re-used so it would still look like it did before, so it would not be to hard or exotic to improve it at home with tools and space to do so.