When my TD was fairly new (built 2006) I bought one of the little Harbor Freight 12vdc heater/defroster units on sale for under $10. As previously stated I wouldn't try to heat a normal size TD with it. I did use it however on a hunting trip to southern Illinois. Overnight temp dropped to 18 degrees-F. I have a 'pull-out' shelf under the cabinets in my TD cabin with a 12vdc 'cigarette lighter' type outlet nearby. About 10-15 minutes before going to bed I pulled the shelf out, plugged in the heater/defroster, & by the time I went to bed I was able to undress without discomfort or being cold. Next morning when the alarm went off I turned on the heater/defroster & within 10-15 minutes I was able to dress, again without discomfort (other than trying to pull insulated underwear & my pants on while lying down

). That little unit actually raised the ambient air temp enough inside the TD to mitigate the cold while dressing. Only thing was after about a half dozen uses the switch 'locked' up & then came apart. Since it was built into the plastic case I couldn't figure out a way to repair/replace it. It had a sliding scale temp control but I just kept it on max (possibly a reason for the switch going south??). Haven't tried another one since; I just 'bite the bullet' & endure a few moments of discomfort while dressing or undressing. my TD is so well insulated that with the 6 1/2" of mattress, a couple of small fleece sleeping bags for sheets, & my 20 below sleeping bag as a comforter, we're perfectly comfortable in any weather we'd feel like camping in...