The diesel contained in the tank at certain temperatures begins to suffer from the cold, the paraffin separates from the fuel and crystallizes, forming real wax plugs, the classic one in the filter. The result is that the engine does not get fuel and the heating does not work, if fueled with diesel, exactly as if the tank were empty. It is important to specify that in Italy from April to October "summer diesel" is distributed , which "freezes" (that is, paraffin crystals are formed) at -2 degrees centigrade. From November to March “winter diesel” is distributed, with freezing at -12 degrees ; to this, in cold areas, it is necessary to add a specific additive, don't do it at the last moment, but give it time (a few kilometers is fine) to mix well and reach every part of the fuel system. In the mountains "Alpine diesel" is available , which resists down to -18 degrees, so fill up at the mountain service stations, then taking care to cover a few kilometers, as for additives.
Don't forget that with snow on the ground, the windscreen will also get dirty more quickly due to splashes of dirty snow mixed with salt, so to keep the windscreen washer system efficient, an antifreeze liquid for the windscreen wiper tank is ideal.
The gas you have in the cylinders (or in the fixed cylinder) is LPG, i.e. Liquefied Petroleum Gas . LPG is a mixture of various gases, you are interested in the two main ones: butane and propane. I have dealt with the subject in depth in this article , but it is worth remembering that in winter, already at temperatures close to 0, what must be in your cylinders is propane . So ask your dealer specifically for propane cylinders. If you have a tank , you have to fill up at mountain service stations, because there the percentage of propane in the LPG distributed is the highest. As long as there is propane in the tanks, you will rest assured, because up to -42 degrees centigrade propane will continue to be usable ( read the article ). Do not cover the cylinders , leave them as they are, without putting on coats of any kind, the propane will continue to "work" precisely because there is heat exchange with the surrounding environment, which will take place up to external temperatures of -42. Have you ever taken a cylinder in a rest area or in a camping in the mountains ? They will bring it to you (generally weighing 20 kg) and you will connect it to your system instead of one of yours, leaving it outside. It will work great, it will be full of frozen frost, and this will more or less also let you know how much gas is still available. In short: no problem with propane, full of gas and diesel to do in the mountains, with almost empty tanks.