In my experience, if you don't crack a window or sunroof, you get condensation on the inside of all your windows. It sucks.
My setup is this:
-Honda Element (the ultimate mini-RV) with seated folded into a bed, -sunroof cracked (the way mine is, snow and rain doesn't get in)-clear the snow off of the roof of your car so less snow falls on you when you open the door in the morning-use the passenger door for the above reason-0* sleeping bag (or lower, I am always warm so I'm fine with 0)-Beef Jerky, peanuts, lots of water, nature valley sweet n' salty peanut bars, gogurt kept buried in snow next to you. -extra car key in your ski pants, coat or your gascap (something that won't be locked in the car)- Cook under the cover of your open trunk (for me I have a tailgate and rear hatch, so I have a covered place to sit/cook on)
If you're cold:-wear a beanie and socks. This solves most warmth problems.-Layer your clothes. I'm usually fine sleeping in long underwear, thin underarmour shirt, ski sock and beanie-don't wear cotton. At all. -Tuck your hands in your crotch or armpits. -breathe into your sleeping bag (until it's warm)
Following all this, I have spent many happy nights in my car.