As others have mentioned, your big problem is not the absolute cold temperatures, but rather the condensation from temperature fluctuations. So, figure out some moderate temperate, say 40 F, and keep the camera steady at that temperature. If it is moderately cold weather, I simply wrap my camera in my warm coat and then I use that as a pillow at night. If it is really cold weather, then I sometimes put the camera inside the sleeping bag. But then, when it comes out, I try not to let it go directly into the cold. Battery packs are easy to pop into your pocket.