I have had good success with foil-buble
insulation.  I have cut panels to fit
the entire poptop...inside for
winter...outside in the desert heat.  It
works very nicely.  It is especially
nice in early spring or late fall
mountain weather when it can get quite
cool when the sun goes down and the wind
kicks up.  You can have the poptop up
and have it be very cozy and warm
inside.  It helps for sure in extreme
weather camping, but without some heat
source upstairs, it's going to still be
cold sleeping.

You can see a picture at www.syncro.org
under camper improvements.