Propane works better than butane below freezing. A butane supply under similarly cold conditions suffers from low pressure (even when there is plenty of gas in the cylinder) and wont work much below minus a few degrees.
Typically caravans and larger motorhomes store gas outside the living area and therefore use propane which is heavier and bulkier for the same heat output.
The MP’s gas bottle is inside the van below the wardrobe and where it should be warmer than outside especially if you have heating on.
If you are camping in conditions where the interior of the van is going to get so cold that the butane gives up the ghost, drain both water tanks and use bottled water.
Connecting propane gas to the campervan supply without changing the burners and regulator is potentially dangerous. It could affect combustion and combustion products e.g. carbon monoxide. Remember, the van should still be ventilated when operating the stove, even if it is very cold outside.
If you are going away only for a few days, converting to propane seems an expensive hassle. Perhaps a small lightweight multifuel stove running off a screw-on canister of propane or propane/butane mix, or a Jetboil would be a backup for a morning cuppa.
we had lived in Alps for 3 weeks inside Marco Polo. I can say that gas will get frozen at -11 C degree. If it is -8 outside, it works fine. Water is not frozen too. But you need to hold in warmth MP all the day continuesly . As result, I can say, that we need to think about how to heat up gas and water in Scandinavia. The weather there is colder than in Alps