A few years back I found a 750 watt personal heater, like the vornado sold at target. On low its about 400 watts. I don't remember if I've used it. What has helped in cold camping (20F) for me was to really warm the interior for an hour before bed. Get everything inside up above 70f. Then turn thing down at bed time. I also hung cheap blankets on the walls, that really reduced the chill and the condensation.
Tom
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