The duct work for the furnace will keep the enclosed area fairly warm to keep your tanks from freezing probably in the upper 20s, but I would be afraid to use too use the camper in the teens.
The big problem is the dump valve system that hangs below the enclosed bottom....there is nothing to keep that from freezing. I wrap a string of rope lights around the dump valve and piping as a precaution.
Another problem would be if you are hooked up to a fresh water supply....that line will freeze. Had that happen back in February, and it took awhile to let the sun thaw out my lines.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263