If I were you, I would go and speak with campground owners and hosts, and local dealers in the area that you are looking at. I would ask the campground owners and hosts what precautions they take to keep their rigs in working condition. You have to remember that unlike a home, there is a big gap between the bottom of the trailer and the ground. I would ask them about skirting and insulation under the trailer, do they have a heat source under the trailer, and do they insulate and heat their fresh water and sewer hoses?
I would also go and speak with the local dealer and see what they have to say about wintering in a fifth wheel in that area. They are the ones that will be able to guide you and help answer your questions because they know the climate there, and they more than likely have customers that are doing this very thing.
I know that there are people in northern Alberta that are working on the pipeline and are staying in fifth wheels. I also know that they have taken extra precautions to keep their trailers working properly.