Wool goes a long way. Wool blanket over my 0º bag has kept me warm sleeping in my car on -10ºF nights in Canada and Wyoming. This is the blanket I use and the highest percent wool blend I found that's still at an affordable price (pure wool blankets can easily be a couple hundred bucks).

I also adapted this tutorial to make insulating inserts for my windows--more heat is leeched from your windows than from anywhere else in the vehicle. Just make sure to trim them as you go to allow for use with a heater that requires window(s) cracked.

I also plan to strip/apply insulation to my entire sleeping area (in my case, the removed backseat/trunk of a car) and put a layer of reflectix/insulating material under the mattress for good measure.

Good luck finding solutions--would love an update on what you end up going with and how works out.