We have discovered that even when the interior of the trailer is in the low 20s (yes, we have a thermometer), we are fine without any heat source during the night -- just a thick down comforter and a couple more blankets on top of that. The problem is getting out of bed in the night and in the morning -- we wear Russian lamb's wool hats and lots of warm clothing. And yes, that is a buildup of ice on the inside of the window behind DW:

How cold does it get? Once, the cord on our coffee maker froze -- this is not a trick shot:

On second thought, that 12 volt bed warmer is sounding pretty good!