Scafool touched on this, but I want to expand...

A van is a large area to keep warm without the aid of a heat source. Heat sources all produce differing risks from CO, CO2, fire, et.al.

Insulating the interior of the van will be important.

I am sure everyone has seen the windshield sun visors that are made from what looks like mylar coated bubble wrap. That product is know by the trade name Reflectix, and is commonly used as a radiant barrier in construction retrofits on single family homes. I use it in the windows of my RV when it is in storage, and it drops the inside temps by about 20 degrees F. It stands to reason that one could buy a roll of this material and pre-cut insulation for a vehicle. One could insulate the interior of the van in an emergency, creating a smaller area to heat at the same time.

I know that once I take delivery of my new truck I will be cutting panels of the stuff, labeling them and storing them for use in just such a possible emergency while I am touring Canada this winter.

This product also works well as a field expedient sleep system when combined with duct tape to make a "sleeping bag".

All the usual disclaimers, but: I have used this product as it was designed, and in my own designs... Hasn't failed to satisfy me yet.