We just got back 2 weeks ago, but could not take the RV.
We were on the south rim, and December was a great time to go. It was not crowded at all.
If you are able take a hike down into the canyon, even if just a little ways, the views and perspectives are totally different. We hiked to Ooh Ahh Point, and it deserves that name. Also check out the different overlooks, they are all different, and are different at different times of day due to the sun angles.
Also stock up on food before getting there, the little stores in the town outside the park are nice but pricey, and the restaurants were not very good.
It is an incredible site, but look at all the other things to see in the area if you have time. Zion, Bryce, Arches etc.