I think -15C is a good assumption. How to get water there? The only ways I can think of is: melt snow, which needs a heat source, or go down to the stream in the forest and drink there. Maybe bring back water, though it might freeze on the way.
And if it's -15 outside, it's going to be the same inside the tent, without heating. Just as cold, only less windy. That they wouldn't have undressed in the unheated tent is not my thought, I'm just repeating it, it comes from Yuri Kuntsevich.