AndyL,
I completely follow you on the Alucobond. One of the products that I have looked at is 3mm Polymetal. The Polymetal has aluminum skins that are a thinner gage (.013"-.015") than the .020" skins of the Alucobond, but the construction is basically the same, a 4x8 panel of Polymetal is about 22#. If you could get Alucobond at a deal, it sounds great. Some weight could be saved if you relied on the Alucaobond for a stiff, durable, prefinished external skin, and used lighter sheet aluminum (or 1/8" plywood) for your inner skin where the stiffness might not be necessary. But even if you used the Alucobond on the interior, it's not that much weight. The stiff composite panels would not need to have wall "studs" spaced as closely as a less rigid panel and that could save some weight. Without doing the math, I would guess that you could do an Alucobond/steel tube/Alucobond stucture with insulation for about the same weight as 3/4" plywood.
It sounds like you are more comfortable working with steel than wood, which is probably opposite of most of the folks on the forum. With a steel body framework you could weld all of your joints then skin the framework. That's been done, you are just looking at a different type of skin that is much more rigid than what they have used and shouldn't "oil can" on you. Oh, and it's prefinished if you can keep from scratching it up during construction. Leave the protective film on until last.
Are you thinking about a traditional teardrop shape with a curved side profile? If so then you will have to consider how you would bend portions of the steel tubing. Others on the forum have done this. But if you were thinking about bending large section, thin-walled tube you might run into problems with it kinking. If it were me, and I wanted to use steel as a framework - just opinion - I'd look at 1" square, maybe 18 gage so that I had a little thicker wall material. That should be plenty rigid for what you are trying to do, unless this will be a really large trailer.
I've played with the Polymetal material and there is very little insulation effect with it. Heat will transmit through the material pretty well, it does not act as much of a thermal break. This would have to be considered in your design. Using a bonding tape to attach the outer skin to the steel framework will allow for thermal expansion differences between steel and aluminum and would act somewhat as a thermal break. It's used often for high-end trailers without additional mechanical fasteners. See:
http://multimedia.mmm.com/mws/mediawebserver.dyn?6666660Zjcf6lVs6EVs66Ser3COrrrrQ-
There are generic bonding tapes that don't cost what 3M's products do and probably work nearly as well.
I'd encourage you to keep looking at your design. There was a recent thread that looked at some of these possibilities, as well as others, that might be of interest to you if you missed it.
http://www.tnttt.com/viewtopic.php?t=23005
J.B.
I've seen the steel framed trailers done, usually using 1" square tube (.100 wall I assume?) My metal shop recently dropped off a catalog - and I notice they list 1 1/2" square ERW - down to as thin as .035 / 20ga.
