For what you describe I would be looking at a used 4WD class C. -20 off grid is not going to be easy if you expect full use of the water system or you will be running the generator for extended periods. Still going to need heat pads on the tanks and pipes. Extra batteries, extra propane, 300+ watts solar etc. It sounds like you want an expedition vehicle.
You can see how it goes in more mild conditions and up the mods or stay away from extreme conditions as time and money permit.
Here you go a 4x4 diesel class C $21,000. Just upfit for cold weather.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29404546.cfm
