We have had a few nights down to -17f. We have a diesel fired furnace that put out plenty of heat, but even with that running and keeping the core air about 70f the issues we found were:
-Around 10-15f or so the floor still gets to be pretty chill, (easily solved with insulated carpet & down booties), but can freeze interior waterlines on the floor.
-With the canvas pop-top, the cool air coming off the canvas walls can make quite a curtain of cold breeze that would prompt us to close the top at ~0f. Much colder and the plasticized canvas and vinyl windows become too stiff to fold up anyway (and perhaps might crack?).
-If the dash vent is left open, a breeze from the front can blow in and cool things down quite a bit in just a few minutes. Some ventilation is good, but with windows you can control it a bit better.
With the pop-top down and furnace on even the coldest nights were quite comfy though.
-Erik