Michigan to California this December

The nice thing about crossing the mountains in the winter (both the Rockies and the Sierras) is the weather does not sneak up on you. The snow comes from storm fronts that move in from the North Pacific or down from Canada. That means they can see them coming a week in advance and can usually predict how much snow they will drop. So just check the weather forecast before you trip and if it looks bad, then take the extra time and go South and come across that way. But there are mountains to cross down there too so watch the weather on them too.

I know others have driven their RV in the snow, I don't think I would do it on purpose. I'd go around or park and wait before snow driving, especially across mountain passes.