I have a lasko 200 watt heater, and a lightly insulated van. It does not accomplish much. High 30's I can maybee maintain 52.
That is plugged in.
On 12vDC battery power, powered by an 400 watt PSW inverter, it is a 19 amp draw at 12.6v, and higher than that as battery voltage drops.
After about 2.5 hours my healthy fully charged 90AH AGM battery would be about 35 to 40% charged, and 4 hours it would be gasping, as best, and somewhere around there it would be unable to hold a high enough voltage where my inverter could continue to power it.
From that point, assuming I stopped before draining the battery completely to 0%, I would have to drive for an hour first thing in the morning with my well wired alternator spinning fast, and then, on a sunny day , my 200 watts of solar might be able to finish recharging to full the battery. before late afternoon. Maybe. On a good day.
So electric heat is not only not very effective, it is a huge battery consumer.
So 'just' powering even a 200 watt electric heater would require huge battery capacity and huge amounts of solar and other recharging sources.
I do use mine, for 15 minutes under the covers to help warm the cold bed, but I never try and heat the van with it unless plugged in, and even then it only takes some of the edge off.
This 12v heating pad draws about 4 amps, and cycles on and off depending on hot hot it gets. It maxes out my well insulated 5 gallon shower bag at 101F, and consumes about 28AH over ~12 hours to get ~65 degree water to 100F.
https://www.amazon.com/ObboMed-SH-4...118?ie=UTF8&psc=1&refRID=STCWK3TTZN5A4GT96GS8
If electric heat is really wanted to be used on battery power, you need to apply the heat directly to your body or bed or seat, and it can still consume a lot of battery power to do so.
If you really want to heat the air, then propane and proper ventilation are required. The Big buddy heaters seem to be the popular choice.
