While I've camped in temps down to the upper teens, I think I'd be fine down to temps near 0.
Like many, my FW was advertised as 4-season. I was impressed, with the thought that went into the design. First thing, everything is very well insulated, noticeably quiet inside, in a busy RV park.
The heat ducts, from the furnace, are next to the waterlines through the various cupboards, and exit above the floor. Other heat ducts run under the floor, still next to water lines, with one duct right next to water pump.
The entire basement, with all tanks enclosed, is heated and well insulated. Even the baggage doors are insulated. All flooring has Refectix insulation on the underside, even in the baggage area under the bed.
After tenting a number of years, this is my 4th RV. It is a Yellowstone brand, which at time I purchased new, was the high end division of Gulf Stream, and built in it's own separate factory. No complaints, most things about this unit were better than expected, when comparing to previous RVs I owned. :)
Jerry