Wife and I were at Yellowstone park area this fall in the Alaskan. Temp was down to low of +10 F degrees at night. Slept fine with (stock) furnace OFF all night, and kick it ON to take the chill off in the AM. We were hooked up to RV park power, so fan use was not a concern. Propane furnace brought the interior temp up rapidly in the small interior space when time to rise.
We use a *Hacky Sack bag to sleep in, on top of a 2" memory foam toper, on top of the Alaskan camper foam mattress, so no cold seeps up from underneath in the cab over bed. We do have a small dog that does contribute a couple BTUs to the interior of the camper...
That is coldest we have slept/"camped" in it so far. Not likely to go colder intentionally, but we both did just fine (retired softies here).
*Hacky Sack (brand) is a big indoor sleeping bag for 2 that when zipped together (on both sides) has a summer side, and a thicker warmer side for versatility. A single Velcroed in place (on just the sides/edges) U shaped looonnng sheet is removable to launder the sheet (bottom of U at feet). We like it because "making the bed" consists of straightening the bag. We would recommend it or similar.
Edit: bought the bag online years ago as a web purchase from small vendor. I can't find any evidence of them now on a google search, so likely out of business? :-( We do love the convenience and versatility of it.
A seamstress could make one pretty easy, and it wasn't cheap. It has held up well so far, but hasn't had a lot of use (unfortunately).