I was (am) having second thoughts about a fifth wheel for my use, as I chase the snow during the winter rather than running from it. Fiver has some issues in the cold, i.e. going down the road on the furnace to keep warm, no engine heat etc. Did a search to find a vehicle designed for winter and found the www.bigfootrv.com
Looked at some of these class Cs in Bozeman Montana and they do some good things when it comes to insualtion etc.
They use spray urethane foam in the walls, ceiling and floor for a R value about 12.
They fill the square aluminum tube frames with foam to help insulate.
They have two floors, with about a 6 to 7 inch gap, where they run the heat and other utilities. Both floors insulated.
They run the engine heat into the holding tank area which is fully insulated, as well as the furnace heat.
They do some good things, worth a look at when you are making your dream coach.