Well, here's what I would do. I don't particularly like this solution, but it's the best I can come up with.
The first thing would be to get a cold weather sleeping bag. That will keep your body warm, but not your head. You probably should get a ski cap for that.
So far I haven't seen any 12-volt heaters with a thermostat. They might be some, but I haven't seen them. If you can find a small space heater with a thermostat, then that might be ok. The thermostat would need to be able to go very low - maybe 50 degrees. This is not the main way to keep yourself warm at night - the sleeping bag does that. The purpose of the heater is to keep the interior air above freezing.
The space heater would be AC, not DC since DC heaters generally don't have a thermostat. That means you would need to get some kind of power generator like a Jackery or Rockpals. You would one with an pure sine wave inverter because a modified sine wave would cause the fan in the space heater to buzz. Also it needs to be able to be recharged from a cigarette lighter plug, and can be charged and discharged at the same time. You would then plug the power generator into the car and the space heater into the generator. Of course you would have the prius in 'ready' mode.
In effect this would be transferring power from the high voltage battery to the 12-volt battery to your power generator. Your engine will periodically auto start when the high voltage battery drops, but it would not remain on nearly as long as when using the car's heater to remain warm.
There is a risk with this solution so be VERY CAREFUL THAT NOTHING FLAMMABLE can get close to the heating element.
I used a similar setup a while back to run my Dometic CF-18 when I was car camping in my Prius. I wanted to be able to keep DC power to my fridge when I went on a hike, but didn't want to keep the car in 'ready' state while I was away. My power generator provided the power when the car was off and automatically would be recharged once I got back.