I agree that it is the wrong time of year. Flagstaff can get something called feet of snow, and while Virginia also gets several inches in a bad storm, at 7,200' elevation 12 inches of snow is likely during late December.
My first RV trip was also to the Grand Canyon over the 4 day Thanksgiving holiday. I saw practically nothing of the canyon, to much fog. I also had no furnace in that camper. So I went to Laughlin, where it was in the 70's along the Colorado river at 900' elevation! That was back in 1987, before weather.com
I would suggest taking the RV from Phoenix to Tucson, and stay at the campgrounds around there. Visit the air force base, and the airplanes around it. Saguaro National Park, with Old Tucson studio's and lots of cactus to look at.
There should also be things to see around Phoenix, check out the local guides.
Fred.
