When our boys were in Scouts & did a few winter camps (in Canada) we found the "foil' survival blankets worked well. They used them under the sleeping bag with another blanket under the 'foil' blanket next to the air mattress. Of course the Scout Leader advised against using air mattresses at all for early spring/fall/winter camps as they are the coldest thing to sleep on but our boys didn't like the thin foam pads.
We have friends with a tent trailer & they have 1/4" sheets of styrofoam on the plywood, under their foam mattress as well as a 10-12" high strip of it along the 3 outer sides of their bed, against the tent sides. They covered the styrofoam with vinyl stickey-back wallpaper/shelf liner from the dollar store to protect it. They said the foam makes a huge difference on cool nights. That wouldnt work on top of an air mattress but maybe the insulating material people are using for winter trailer camping would help.