Are these cost effective and sufficient for keeping warm with the engine off? Say down into the teens, above the point where diesel gels up. Or will this kill your battery if you're able to get 8-10 hours of bunk time?
The the caveman days before Espar, I used a 12-volt electric blanket. The next winter I got an Espar heater. Now I sleep very comfortably and warm and toasty on top of the covers, usually nekkid but sometimes while wearing socks, no matter how cold it is outside.