Camping at the South Rim in December will be cold cold cold. As someone who lived and worked there (on and off) for three years, I can definitively state that. It will be cold in December. Looking at weather maps of the state, the South Rim is consistently the coldest overnight temperature in the state. Their overnight lows are already in the 20s. Of course, I have no data on the White Mountains, since the news never bothers to report on them, nor the Lukachukais, but even if Alpine gets colder, it doesn't really matter since you're not thinking of camping there.
Just had to throw that out there.
"The only thing we did was wrong was staying in the wilderness to long...the only thing we did was right was the day we started to fight..."
My book, The Marauders on Lulu