I've been out a few times when temps dipped to about 25 degrees. Not very cold but chilly and below freezing.
The furnace in my XT276 kept the inside toasty warm and very comfortable with no need for an auxiliary heater...electric or propane although the floor was a bit cool, no problem with slippers/shoes on. The XT276 has no 'underbelly insulation' .. just the plywood floor and floor covering of linolium although we do lay down a carpet in the living area ~ mostly to make it more 'homey' per the wife.
The 2012 XT-276 comes with a 20,000 BTU forced air furnace that is more than adequate for it's size.
PS: to calculate the BTU's need to heat/cool and area, check this chart.
2012 Fun Finder XT 276 Toy Hauler & Harley Davidson Ultra Classic with Condor wheel chock
2014 Dodge Ram 2500 AT 4X4 Crew Cab 6.4 L Hemi, 373 gears, tow package, Rambox option, Revolver X2 tonneau cover, Equal-I-zer anti-sway hitch.