You are absolutely correct, but now your life might depend upon whether the battery and its connections are all functioning properly - that is adding complexity. My personal feeling is that I would rather keep it simple and tuck myself into a nice warm bag, just like I do routinely when camping in winter conditions. Simply regard the vehicle interior as a glorified tent (no flapping in the wind or collapsing under a snow load, for example). Nights when I have bivouacked in my vehicle during wintry conditions have been rather pleasant. I have never had any difficulty is sleeping soundly and arising well rested.