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

If you are winterized, no shower, no kitchen faucets, no onboard water. You can use the toilet -- carry a 5 gallon jug of water to flush/wash, and be sure some antifreeze is in the tanks.

There are some year-round campgrounds/RVparks. Lakeside RV in Provo is one I've used. So dumping is available.

RV'ing in winter is tougher than summer. If I were you I'd be sticking to the Arizona route.