That's a tough call. Nothing is going to be fool proof, especially with an outside shower, which many units have.
At a minimum, you need an enclosed underbelly. If you use the furnace, it should keep the sub floor area warm enough, but that's a lot of propane you will use.
We've camped during a few hard freezes in the Winnebago with no issues, but into the 20s or below freezing for 12hrs+ and your asking for trouble. Many of the units that might be specially set up for this are not sold down here. See Arctic Fox.
2013 Winnebago 2301BH-Red
2012 Ram 2500 Megacab HO CTD