OK, here comes today's lesson!
There are two sorts of curvature (of a surface) - single curvature and double curvature.
In single curvature, the surface is curved in only one direction - at right angles to that direction, the surface is flat. Examples of single curvature surfaces are cylinders, cones and teardrop trailer top skins. A flat sheet can be bent into single curvature - if you let go of the sheet, it will spring back, or can be bent back, into a flat sheet.
In double curvature, the surface is curved in both directions and there is no direction in which it is flat. Examples of double curvature are spheres, old car fenders and (horse) saddles - note that in the saddle, the curves go in opposite directions, one inwards, one outwards. A flat sheet cannot be bent into double curvature without stretching and/or shrinking it in places - if you let go of a double curvature sheet, it may spring back a bit but it will not go, or be bent, back to being flat.
Now that trailer model requires lots of double curvature and you just can't bend plywood to do that. You can't even bend an aluminium skin to do that, unless you've either got a big press and dies (and a spare million dollars or two....), or you can shape sheet metal on an English* wheel.
The two materials/construction techniques that come to mind for low volume, thin double-curvature are fiberglass and cold-moulded plywood. Fiberglass is only a way of reproducing a double-curvature shape - you still need to make the original plug or mould.
Cold-moulded plywood involves laying hundreds of strips of plywood, many of them tapered in width, over a mould in two (or more) layers. When both layers are completed, the whole skin can be lifted off the mould.
I'm sure I've shown some of these photos here before, but they are so stunningly beautiful, that they bear being repeated. This is a one-off cold-moulded wood monocoque (=unibody, =frameless) three-wheeler car body. The last photo shows the mould over which the body was laminated.
Like building a one-off in fiberglass, this is no more than ten times the work of building a normal teardrop shape in plywood. Well, OK, maybe twenty times but, anyway, a lot less than a hundred times......
So now you're going to ask how an Airstream can have double curvature.... and I'm going to say that it doesn't (mostly). If you look closely you'll see that most of it is just single curvature within each piece - like a football, the pieces are stitched together to make something that looks like it has double curvature.
The one exception is those quarter panels under the front marker lights - those may be pressed into double curvature, but actually I doubt they are - I think Airstream has just carefully worked out how far they can go with distorting single curvature panels.
Andrew
* Strangely, we don't call them English wheels - but then we don't have anything like an English muffin, either!