battery failure has been a big worry for me when we camp out , I deal with it by having a smallish truck battery that is usually on a solar trickle charger when at home ( gentle charge , keeps it topped up but doesnt boil the water out )
that battery travels with us in whatever vehicle we are taking bush .
Its enough we can start the big diesel in the bus as long as there is SOME kick in the original battery .
Itll easy kick over the smaller petrol van.
having the second battery saved my backside a lot of times over the years and helped out a lot of other folk I met as well . Same as the can of fix a flat , and the 20 litres of waster and can of bars leaks that ride along "incase" as well have been called on a couple times a year , usually to fix other folks problems but occasionally my own too .
I have done a crude version of the battery warm up to make a dead battery work , a couple times , just sitting it next to the fire , moving some coals aside to give it some hot dirt to sit on , wait till its a bit too hot to touch but the plastic isnt melting quite ... wrap it in a t shirt or something to carry it back to the car and away ya go . it does work , but has potential to make a bit too much fumes near the fire and explode on ya too , so I hear anyway .
The cold here doesnt get desperate where I go , but wrapping up like you are a homeless person sleeping on a park bench works for the bit of cold we get , ends up being you have a toasty warm cocoon to sleep in , even tho everything else is freezy cold around you . My experience anyway , I been both the homeless guy , and the guy sleeping in the car .
them silver foil and cardboard reflectors meant to save your car interior from the sun make a good reflector that keeps your heat from all escaping out the windscreen too , covered windows shed heat slower than exposed ones it seems to me anyway .
the bigger hassle here can be heat in the summer , sleeping in the car provides some protection from snakes and bugs getting into your gear but if the car is closed up in day time and youre in it having a snooze , the interior temps can get pretty high , enough to cook eggs in their shells left on the dashboard , just not quite hot enough to make them explode .