I'm an aerospace engineer too. I'm also building my original-design plane
I used to work at Boeing. They used to have Boeing Surplus. That place was a TREASURE TROVE for folks like us! They had the honeycomb flooring out there for cheap. Too bad the place closed up a few years ago. That stuff new would cost more than the rest of the trailer. But there, it was pennies on the dollar.
I'd designed a totally new frame for my 31' Excella. I was going to make it 8" deep. Get a much bigger "I" and all the SAG problems go off into the sunset. I also don't by the myth that these trailers are monocoque...the cutouts are too big.
Because I was building my house at the time, I didn't have time to build the hoist mechanisms to pull the shell, roll the old frame out, and roll the new one back in. So I sold the Excella and got an Avion 34X. The biggest difference between Avion and Airstream that I see....Avion's frame is much heavier and the shell sits on a perimeter frame attached to the outriggers and the floor sits inside. Sound familiar to some of the "better ideas" on this thread??
The one gent had what I consider an excellent notion: Do the perimeter plate and let the skin attach to that; have a 1/4" gap, and have your treated floor inside that. Gives the water a place to drip through without messing up the floor. Heck, use TREX or something for the floor if you can't get the aluminum honeycomb.
Another trick Avion did was that they put linoleum on the floor, wrapped it completely around the edges, and stapled it from the bottom. In other words, you cover the top surface, wrap around the edges, and have a 3" overlap on the underside. That way if/when water does get to the edges of the floor system (which is actually two sheets of 1/2" plywood with 1" staves between them....think I-beams, with foam insulation between the staves), it can't wick into the wood.
Don't get me wrong, I really liked my Excella (even with the dual pane Vista Vue's that leaked like a sieve and the Scotch Tint dissolved into crumblies that looked like somebody poured their smashed up frosted flakes into the space between the panes). If I wasn't in the house building mode, I'd have built a new frame.
Indeed, I'd still like to get two of them, cut off the front of one and the back of the other, and build a 40 footer with a couple of slides. Might do it one day.
But if you're going to this length, I like your idea of setting the skin on its own thing and putting the floor inside it.
We Eng-I-Neers tend to think logically alike....usually
Best of luck,