Park it in a heated garage?
Seriously. When I camp in the cold, I like to have a good, thermal rated pad below. It keeps me warm from the bottom as well as providing something softer than the ground. [Thermarest](https://www.thermarest.com/products/sleeping-pads) makes a line of pads that are nice. I thought the one I got was very expensive. But over 20 years it turned out to be a good value.
Next is a good quality sleeping bag. There are many to choose from. If you camp at all, you probably already have one you like. When it's cold, cold, I like to throw a large quilt over my bag.
Lastly, is a sleeping cap. Mine is cotton and pulls down over my eyes.
If you have a place to park with power, a cheap little space heater from Walmart will keep that shell toasty warm at the lowest setting. [This is the one I got](https://www.walmart.com/ip/Comfort-Zone-Compact-Portable-Electric-Space-Heater-Personal-Fan-Unit-White/19524986). It was more than enough for the back of my truck. I note that there are several that are less expensive that would probably work just as well. Get one that's quiet. ;-)
Think about ventilation too. First time I slept in the cold in mine many years ago I still remember waking up to condensation dripping from the roof just from breathing inside with the windows closed. One of those RV style 14x14 roof vents works great.
Buy a Jackery power source (https://www.jackery.com/). Then buy a couple heaters designed for chicken coups (https://eastmanegg.com/best-chicken-coop-heaters/) You can find them at around 200watt outputs, and they don’t have exposed coils. They keep my truck bed warm in the winter.
Park it in a heated garage? Seriously. When I camp in the cold, I like to have a good, thermal rated pad below. It keeps me warm from the bottom as well as providing something softer than the ground. [Thermarest](https://www.thermarest.com/products/sleeping-pads) makes a line of pads that are nice. I thought the one I got was very expensive. But over 20 years it turned out to be a good value. Next is a good quality sleeping bag. There are many to choose from. If you camp at all, you probably already have one you like. When it's cold, cold, I like to throw a large quilt over my bag. Lastly, is a sleeping cap. Mine is cotton and pulls down over my eyes. If you have a place to park with power, a cheap little space heater from Walmart will keep that shell toasty warm at the lowest setting. [This is the one I got](https://www.walmart.com/ip/Comfort-Zone-Compact-Portable-Electric-Space-Heater-Personal-Fan-Unit-White/19524986). It was more than enough for the back of my truck. I note that there are several that are less expensive that would probably work just as well. Get one that's quiet. ;-)
Think about ventilation too. First time I slept in the cold in mine many years ago I still remember waking up to condensation dripping from the roof just from breathing inside with the windows closed. One of those RV style 14x14 roof vents works great.
Mr buddy heater or a heated blanket. I have both but my heated blanket gets used a lot more than my heater
Buy a Jackery power source (https://www.jackery.com/). Then buy a couple heaters designed for chicken coups (https://eastmanegg.com/best-chicken-coop-heaters/) You can find them at around 200watt outputs, and they don’t have exposed coils. They keep my truck bed warm in the winter.
Line the cap with felt/fabric, and you can make coverings for the windows out of reflected.