The Climate in which I live is mild so Perhaps I should avoid this thread, but here are some strategies I employ.
I have a 200 watt lasko heater. I can run this on my inverter,( ~18 amp load) but it is so ineffective in my lightly insulated van I only use it on battery power/ inverter, under my covers, knees tented, for 10 to 15 minutes to help warm the bed when i first crawl in, Naked.
If I would sleep in my clothes, or just take them off under the covers after warming the bed a bit, this would be unnecessary.
If I am plugged into the grid this 200 watt heater run continuously can perhaps keep the lightly insulated van 10F warmer than outside. But on battery power, 18 amp load is way too much for my battery capacity. Accounting for Peukert, I'd be below 50% in about 2 hours, if that were my only load, which is is not.
My roof vent has a 4 inch diameter aperture. When Cold I turn off the 120mm computer Fans, Pull the second fan down ( magnets) and install a napkin or 3 to act as a barrier to prevent hot air from easily escaping.
Sometimes I have forgotten to remove this napkin when making coffee with my Bialetti Moka pot in the morning on my propane stove, but as soon as it starts steaming I realize the roof vent is still covered as the steam collects a the roof.
Have I ever felt any degree of Co poisoning during these forgetful events? No. But I try and avoid them.
I have a 12vDC 45 watt heating pad designed for heating car seat bottoms.
https://www.amazon.com/ObboMed-SH-4...58674&sr=8-1&keywords=obbomed+12v+heating+pad
mostly I use this to heat my Shower bag, or when my lower back acts up, but it could be used undercovers or on my third chair to warm my body. It will cycle, and eventually get my 5 gallon shower bag to 100F . The ~ 4 amps it draws( wattage depends on voltage it receives) is not continuous, and AH consumption not really extreme when I lay on it under the covers.
I have never used my propane stove solely with the intention of heating my Van, but again, perhaps if I were in a colder climate I would.
This 12v 45 watt heating pad, heats my Shower bag fairly quickly when I layer my jackets or other clothes atop it. As long as I keep the shower nozzle above the shower bag it does not leak and is unlikely to. I have pulled it onto my lap or under the covers with me when I was cold and feverish and it was awesome.
On cold mornings, I find it best to pull my clothes under the covers with me to warm them up and then pull them on under the covers, then getting out of bed is not a shocker, but again this is only as low as say 35F and nobody has ever accused me of having s swimmers build, or being a cold weather wuss.
I had thought of using a coleman catalytic heaters and mounting it near my roof vent aimed downward. These heat by IR radiation, they do not directly heat the air, but searching for a link on Amazon, I no longer see them offered.