Winter parka and sleeping bag. (maybe 2 sleeping bags)
Insulated pad or layer to keep from losing heat thru compressed sleeping bag material while lying down.

I chose to let myself get snowed in at 11,000' while on a Sept 4WD trip. Basically slept out the storm in my Jeep.

When well equipped there's nothing to fear but fear itself.
(But that sure expanded my personal boundaries!)

Getting the interior de-iced so I could drive out required direct sunshine and time. No way was interior heat going to thaw everything that needed to be thawed.