The rig was awesome in those temps. I was happily surprised. In fact, I was mostly using the the electric heat pump (Air Conditioning heater), but the propane furnace would kick on at times to make up the difference. But we never felt cold inside. I don't have a heated or enclosed underbelly but that didn't seem to matter. We did put a blanket over the cabin to keep the cold air out, as those windows and floorboards aren't insulated. Worked like a charm. Here's a pic of that.
2015 Jayco Greyhawk 31FS-"Scamper"
2015 Subaru Forester 2.5i Prem. 6 Spd
Blue Ox Aventa LX Tow Bar 2014 Jayco Jay Feather X23B-"Glampy"(Sold)
DW, DS, DDx2, and Bandit & Fletcher