Dave,
A lot of ideas for you here!
As far as your recent insulation question, the insulation with a foil facing is meant to be placed in an attic, on the attic "floor", with the foil facing up. Then it insulates the interior ceiling from the heat buildup in the attic and also reflects the radiant energy. The reflective surface needs a dead air space above it to work. Not sure if the foil provides any benefit when the attic is cold and you are trying to keep the house warm...
You best bang for the thickness, insulation-wise, is polyisocyanurate rigid foam panel ("polyiso"). I think it usually comes with a foil facing. Not sure if the Home Depot stuff that you've found is the same thing. Johns Manville (
http://www.jm.com) is a big manufacturer of polyiso (check Larry Sorenson's Outback page, I think that's what he used on the sides of his). Check your local insulation or roofing supply houses in your area.
Polyiso has an R-value of around 7.2 per inch (though it declines a bit with age), So it would probably be your best bet for not reducing your interior space too much and providing good insulation values.
I used Dow Styrofoam extruded expanded polystyrene (XEPS) ("blueboard"; almost identical to the Owens Corning Foamular ("pink board")) in my trailer (R-value of 5 per inch): 3/4" for the sides and 1.5" for the roof. I used this material even with the lower R-value, because I needed something that was structural. But if you just need insulation, the polyiso is probably the way to go.
Two things for you to check about polyiso: 1) outgassing issues (toxic at all?) and 2) how strong is it? (you don't want something that gets big dents from your thumbs every time you move it around or bang into it.
Maybe you could try to make some ceiling panels out of the polyiso covered in fabric and velcroed to the ceiling and see how that works. Maybe you won't need to do the sides at all.
And check into getting some foam mattresses made. At least 3" of poluyurethane foam mattress is going to keep you a lot warmer than the Thermarests you're using. Save those for tent camping...
Hope this helps.
Rik