We have camped down to -15 C (5 F) with no problems, the furnace kept everything
from freezing. We even ran out of propane (long story) at -7 and by morning we ran
into town filled the tanks came back hooked everything turned on the furnace,
and although the water in the bathroom was down to a trickle once the place warmed
up all was well.
As for the holding tanks we put RV anti freeze in them, that kept everything
liquid enough to dump. :