Hi Siegfried
Good advice above so I won't repeat any. I'd leave my bag outside too.
A lot of people seem to worry about using cameras in extreme conditions, but in my experience cameras (like people) are more robust than we think. I generally get about a weeks use from a Nikon DSLR battery (D700/D2x) in cold weather, maybe a 2000 shots, although that is a rough guess as I've never accurately recorded it.
It is worth having a spare camera/body along in case of failure. This doesn't have to be as good as the main camera, but even a simple PnS is better than missing the moment totally.
That said, try to keep gear to the minimum as extra weight can be particularly tiring in extreme cold.
Regarding the sleeping mat I use a couple of tricks to keep warm when sleeping in the cold. An extra layer under the mat helps. A plastic or foil bivi bag is good. Using spare clothes, a rucsac, whatever comes to hand that can add extra insulation helps. I also will boil up water before bed and put this in a Nalgene bottle to use as a hot water bottle. The water is generally still warm next morning so can be drunk to help avoid dehydration, rather than cool/cold water that at least in my case, isn't absorbed by my body as well. This bottle trick has saved my bacon a couple of times in Scotland when late spring subzero weather struck when I was lightweight camping with a thin summer sleeping bag.
Thin gloves worn under main gloves also allow the cameras fiddlier settings to be done without flesh being bared.
A good nights sleep, and warm clothes are critical, as if we are cold then we can't concentrate on the photography no matter how well the camera works.
Sorbo style micro fibre clothes i.e. the micro fibre clothes that look like towelling, generally work better at clearing condensation than the smooth micro fibre clothes that are often sold to photographers. Of course if the condensation freezes ....
