I see what your Wheels and MDianna is right; a bag in a stuff sack doesn't lose warmth per se, but the insulation quality is diminished because the stuffing clumps together causing cold spots.

With your unique situation and desire to stay put in the van have you considered a blanket system? I have no idea the actual warmth property's a GI wool blanket (et al) but they take up little space in the van and can keep someone quite warm. Add a lambs wool layer to sit on and you could stay quite warm.

BenJammin mentioned a few months ago some quilt/bag he uses that has some waterproofing qualities. I remember looking at the link but it was too heavy for what I wanted. It may be great for you.