uses two layers of reflectix. Seems increasing to 4 or 6 layers wouldn't be a problem. That should get you to R 5 or 6 which will be plenty for such a small trailer volume at 30 degrees. Condensation will be a serious issue. You will need either lots of ventilation or a dehumidifier. The dehumidifier would be best and has the advantage of adding heat to your rig and the disadvantage of chaining you to shore power.
Some way to add double glazing would help a lot. I think I would try a poly film over the windows
on the inside and maybe the outside too. Sitka rain may make the outside problematic.
Foam comes in two major varieties, open cell and closed. Open cell is like a sponge and soaks up water (bad), closed cell is impervious to water (good). You probally want to choose the low expansion type, depending on where you are applying it.
Good luck on your move. I hear that part of Alaska is beautiful. I've traveled 49 states, but not Alaska.