Ok, all is not lost. Not being there to see it, we may have to make a few assumptions, but it can be fixed up enough to stay in (don't expect it to be pretty).
The Aerolite is supposed to have aluminum framing for the walls and ceiling. That gives you a starting point. Let the ceiling come down and remove it. Examine the walls to see if their foam is similarly water-soaked. If so, the foam should be dried out or, if too far gone, removed.
Next step depends somewhat on what you have left of the outside and if you've removed the exterior skin or not. If the filon was trashed (I'm assuming it will be), replace the foam with XPS, extruded Polystyrene (Lowe's carries the blue stuff, HD has the pink... same difference) and attach exterior grade plywood on the outside. Foam can be adhered to the plywood with Titebond II, and gaps filled in with Great Stuff (foaming insulation in a can). For the ceiling, you can place the plywood first and then glue in the XPS.
If you need to join sections of plywood, glue and screw them. Butt joints probably need an overlap piece, but 90 degree joints can be pocket screwed (Kreg jig). Why XPS? Well, it doesn't really soak up water... unlike EPS and some others... and XPS has good rigidity.
Now to waterproof your plywood. Take a cue from the boat builders. Get enough canvas or some other medium- to heavy-weight cloth to cover everything. You'll need a few gallons of Titebond II. Dampen the cloth before starting, and it will shrink and tighten when it dries. Start your cloth and glue down low so it overlaps the lower frame, and work your way up on the sides with the Titebond, thinned with 5% to (no more than) 10% water. As you press the cloth into the glue it will not want to move any more, so try to keep it straight and unwrinkled. Go all the way up, over the top, and down the opposite side with your glue and cloth. Move the the next section fore or aft and do it again, with a few inches of overlap. After it's all on and dried, apply another thin coat of Titebond overtop the cloth, working it into the weave. After this dries, paint it (Glidden Gripper works well).
The interior XPS, where showing, can also be gone over in this manner, but rough up the foam's smooth surface first with 60-80 grit sandpaper before adding the glue and cloth. Once painted, it will look quite decent.
XPS comes in various thicknesses. The big stores usually stock 1/2", 1", and 2". But if you want 4" for the ceiling (lower heat bills and more comfort), you'll probably have to order it in. Not a big deal, just a couple of extra days to get it.
There's actually a group of tiny-trailer enthusiasts who use these methods to build entire trailers. Many of the trailers they build do not have any wall/ceiling framing, yet the cloth-to-foam-to-plywood construction proves strong enough to tow at normal highway speeds and withstand decent snow loads. Plenty of info here if you care to wade through it:
Foamies on TNTTT
