Insulation
Reduce, reuse, recycle -- in that order. Good insulation will help us reduce our energy needs and keep our home comfortable and efficient.
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Reflectix
I installed this stuff underneath the subfloor to provide a radiant barrier below the floor as well as help insulate the subfloor from the frame. I also installed it in the top curve of the inside of the walls, attached to the exterior skin with the same foam tape as I put on the ribs. See my discussion of Reflectix here and here. There's a lot of debate about whether this stuff is worth the money, and I didn't exactly use it the way the factory recommends, but I think it's useful for my purposes. I don't think I would suggest using this for your only insulation, but a lot of folks have done just that, and I can't really comment on how it actually performs when assembled that way.
Rockwool
Highly recommended for this application. Check out this post if you want to see the beautiful insulation in my endcaps. Also get yourself an insulation knife and some spray glue.
Use a dust mask with the insulation and spray glue, and be careful about running the fans -- the glue overspray ends up on the fan screen and blade and it's not easy to get off.
Rib Insulation
I insulated the ribs with 3/4" wide, 1/16" thick double-stick foam tape stuck to a layer of pink foam sill gasket. I got the idea from blogger Reniergirl who used 1/32" thick tape. She stuck the tape to her interior skin. I decided I was better off not gluing my skins to my ribs in case I ever needed to access the walls again, and thought the additional insulation of some sill gasket foam would be beneficial.
No matter how good the insulation in the walls is, the ribs will always conduct heat quickly between the inner and outer skins. Adding a thermal break to separate the interior skins from the ribs should help prevent direct heat conduction through the walls.
My controlled scientific study is pending, but I think this is one of the best things I've done to improve the insulation in the Airstream. So far, our single 9,500 BTU air conditioner has had no trouble at all cooling our 27 foot coach in 100 degree heat and direct sunlight.
I used the same foam tape to loosely attach the Reflectix to the walls and ceiling, and epoxied the sill gasket to the top and bottom of the gussets under the subfloor to help insulate the subfloor from the frame. 5 rolls was just enough for everything.