We have done a lot of boondocking in very cold weather -- right around the freezing mark, and often below freezing (but never below zero). We have thought seriously about the Mr. Buddy or Wave heaters -- they are safe if used properly -- but we just don't need it.
We run the noisy furnace during shower time -- but otherwise we do without heat completely! We wear heavy clothing during the mornings and evenings -- DW puts a blanket on her lap during mealtime. At night, we use a down comforter and sleep like hibernating bears -- much better than we do at home. It does get into the 30s inside the trailer. That is not a problem -- it's a good thing!
I know this seems extreme, but we go camping in order to get away from the noise of civilization, including the sound of a heater. And we also value the pure air when camping, so the fumes of a Mr. Buddy are not something we want in our air. (I know it is odorless, silent, and safe -- this is just our choice. There is nothing wrong with those heaters -- lots of people use them safely.)
I should add that we have two group 31 batteries at 110 amp/hours each. This is probably overkill, but that's what we have decided to do.
