Arizona has two zones - north of Phoenix you run into the Mogollon Rim. Along the I-40 corridor you're at 6000 ft. or higher and will get snow in the winter.
The areas around I-10 and south are much lower in altitude and that's where you'll find the warm winter weather.
Like COMer said, the Yuma -Quartzsite area is warm, in fact any place along the Colorado River in western AZ and into CA has warm winter weather. You can explore up to Lake Havasu City and it's London Bridge, Bullhead City, AZ and the casinos across the river in Laughlin NV while staying warm. Same holds true into California, Palm Springs, Joshua Tree, even San Diego on the coast is nice.
Instead of tying yourself down to one location, why not explore a bit? There's plenty of room unless you're looking to stay in Phoenix for the season. Along the Colorado River, the only time things get a little crowded is during the latter part of January, and that's a good time to join us and try some desert boondocking around Quartzsite. Take a look at the Quartzsite Rally threads for more information.
