Very rigid temperatures, prolonged stops (read skiing holidays) and "bare and raw" external tanks do not completely get along. There is no real middle ground, the tanks of the gray and black ones, for those with a nautical background, must be insulated and heated .
I have had motorhomes with non-insulated tanks (and for two or three days it is not a big problem, as I explained above), motorhomes with insulated and heated external tanks via a derivation of the heating system ducting, motorhomes with insulated tanks and heated by an electric resistance .
In my opinion, the best solution is to heat the recovery tank via a derivation of the heating system, the hot air jacket that is created around the tank will protect you from any inconvenience. The electrical resistance , again in my experience, is not equally reliable, it absorbs current and already at a few degrees below zero it may not be sufficient, however it is effective for the discharge guillotine , because heating it will allow it to open (just turn it on a few minutes before of the exhaust).
The insulation alone already solves part of the problem , considering that the water that goes to the tank is often hot and therefore its temperature will be partially maintained. In practice it is a matter of lining the tank externally with insulating and light material and, if you want to use a heating diversion to bring hot air, leave a small gap between the tank and the lining. I have never tried to insulate a tank directly, my campers were already born with this feature, but I would probably opt to have it done by a service centre. Features I would ask for:
complete coating which however does not interfere with the drain valve and with a "minimum" overall thickness, in order not to lower the height of the camper from the ground
creation of a small deviation (I mean with a small diameter) of the ducting to bring the heat into the interspace
effective anchoring of the "box" created, there must be no possibility of detachment
a solution to heat the guillotine (or the valve), but without carrying too much heat to avoid deformation