I've cold-weather camped using just sleeping bags and tents. There is no substitute for dressing for the weather - a lesson most people have never learned. In the winter you
must to dress to stay warm, stay
dry, and cut wind loss. In the winter a hat is not a fashion accessory, it is a piece of sleeping survival gear that is
more important than coat or gloves.
As far as I can see the only
thermal benefit of sleeping inside an Element in winter with the engine off is that it provides a wind barrier. The downsides are that the rigid metal/plastic body is a better heat sink than several layers of fabric and air and if you keep the windows closed, humidity and condensation buildup is quite rapid.
Extending what I know about tent camping in the snow, parking so that there's a snowbank/ wall/ hedge or other windbreak on the upwind side of the vehicle will reduce wind-chill effect, as will restricting airflow under the vehicle body. It sounds counter-intuitive, but if the temperature is below 20F, covering the top and upwind windows with snow will retard heat loss (snow:R-1/inch, glass: R-0.24/inch).
I don't think much of the idea of using Reflectix on the windows. Trapping heat close to your body is much more effective than trying to insulate the 100+ cu ft interior of the Element. A temperature-rated mummy bag is a good start. Adding poly-foil space/emergency blankets will improve blocking heat drain, and they are very inexpensive. You should already have at least one in your winter travel emergency kit. Rigid, foil faced expanded plastic building insulation panels should work well as sleeping pads. ( Foil-faced polyisocyanurate rigid panel (Pentane expanded ) initial R-6.8/ inch)
I've looked at the feasibility of using a catalytic heater to heat an Element as a day shelter, but haven't found a simple way to safely do it.
http://www.youtube.com/watch?v=sfAD0wmIUl8
http://www.youtube.com/results?search_query=+winter++"car+camping"&search_type=&aq=f
One overlooked aspect of winter camping is bathroom capability. Each time you have to get out of your warm, insulated sleeping facility in the middle of the night, you'll pay for it with heat loss.
It's best to go to bed slightly dehydrated, with an empty bladder, and sip
very small amounts of liquid periodically (just enough to make up for transpiration loses) only when you need it.