Note: the furnace has been removed from the Caravel
This past weekend, going from Colorado to Vegas by way of I-40, for two days the temps never got above 20 degrees F. Late night temps as low as 1, and maybe lower in the wee hours. Did one night with a Mr Heater propane heater on low--very comfortable, and another at a hookup with two ceramic heaters on low--very comfortable also. Have found since then that one ceramic heater on low does fine down to something below 15 degrees (recognizing that the Caravel
is a small living space), but I used to do the same with the Overlander and one ceramic heater on high.
The big concern is to remember to pour a little antifreeze in the sink drain every night. My Caravel
has a lower drain trap, which takes care of the shower drain, but it's in line with the drain from the sink. If that trap freezes, it takes a little bit of magic to get it thawed out if the outside temps don't ever get above freezing (box, duct tape, propane heater).
If you've got copper pipes, like I do (and which will soon be PEX, just like in my Sovereign), and they are back in the rear access space, I open the pipe drains if the outside temp is going to get close to 20. Above 25 I don't worry too much, depending on some heat migrating into that space from the living space.
One lucky thing--the toilet wasn't competely drained and the water inlet valve was frozen solid to the point that the foot lever would hardly move at all. I just knew the valve was toast, but it finally thawed out and isn't leaking. After freezing that valve and busting it three times in a row (not all in the same AS--hey, old dog here, no new tricks) I was sure it was a goner.