I recently finished a skiing trip in my Hymer with an alko chassis - which means I have a double floor and tanks in that heated gap. The issues for me didn't get close to freezing tanks! The real issues were electrical power, gas, fresh water, and finally, the loo.
We camped in a car park, without the benefit of a power hook-up, for 8 nights. Had the car park not been reasonably horizontal, that would have been an issue as well!
Fortunately my daughter was repping and we were able to get water from her and empty the loo (although its amazing how helpful complete strangers can be when threatened with a full cassette!) Emptying the grey waste was no problem as its very dilute and isn't any worse than salted water...
Recharging the battery was the real issue and I found that the charge current was limited to 10 amps which means that we would have had to run the engine for an hour just to put back an eighth of the capacity. We actually ran the engine for about 3/4 hour max each day. As an engineer, I believe there is NO issue with the engine whilst doing this - the environment yes.
The gas was a worry but it was OK in the end. We used less than 19kg. I will be investing in TB Turbo refillable tanks soon.
I will not put my skiing kit in a cold storage area at night. I hang my kit in the shower to dry out and get warm. I put the skis underneath in a container where they were fine.
130 litres of water ran out really quickly and I will be thinking of ways of dealing with that for the future. The loo lasted about 2 days and whilst a second cassette is interesting i think we can survive by using public loos as much as possible. In the summer you can always use the green additive and bury it if you are careful to choose a suitable place (I know this isn't popular but after living with a sceptic tank for 20 years I reckon its ok if you are reasonably thoughtful about it). In the ski resort, in winter, its a question of finding the accessible loos.
As I already said, for me the main issues are power, gas, and water. Power is the difficult one to crack. All solutions appear expensive, but I am working on it...