I personally use electric heat. This works really well especially with any amount of insulation. Many tents will have some insulation. Any camper of vehicle will have at least some degree of insulation. A little Reflectix goes a long way.
If you have access to the power grid, then electric heat is a no brainer. Even in an uninsulated tent, a small electric heater will go a long way. I’ve camped comfortably in a tent in ten degrees of weather and snow, and a small electric heater was plenty to keep in nice and warm inside the tent.
I’m using a 2kw battery in my custom diy camper which is plenty to keep it warm inside through the night using a small ceramic electric heater. I charge the battery every day using solar panels or a generator.
Notice how the snow is all melted around the camper. The sun during the day is plenty to charge up enough battery power to keep the camper warm enough to melt the snow around it.
If you’re camping in a van or car, then the vehicle’s heater will likely be plenty to keep it warm on cold nights, but again adding a little reflectix will go a long way.
A Cheaper Option
If you’re looking for something simpler that will work with far less power, then check out electric blankets. They can run off much smaller batteries while producing heat and delivering it straight to your sleeping area. I use both methods myself. But many people use just an electric blanket with a smaller battery such as a Jackery. These small batteries can easily charge from your car’s cigarette lighter or from a solar panel, and they are more than capable of powering an electric blanket.