I spent five years in a scout troop that camped every month year round. My favorite trips were the winter ones. I would absolutely get rid of the airmattress. if you can double up on the foam mattress that would be great. If not do you have any "excess" pine trees? pine boughs work beautifully. Or how about 6-8 inches of pine needles? And maybe wear some thermal bottoms and a knit cap? (if that was allready in the list of items worn ignore me) This should really help out.