Renting RV Late Dec. - Phoenix-Denver...All Routes Unsafe?

Be aware that you might need chains in some areas. It's the law.

I, too, would cancel the RV and motel it.

However, I'm not sure why you're planning this particular trip. Are you flying into Phoenix and flying out of Denver? Can you change your plans to fly into Phoenix and out of Phoenix or even Tucson. There are some wonderful sites to see in southern Arizona without getting into winter-type weather.