As others have said, it will be cold -- maybe very cold because of the high altitudes of most SW national parks. The major Pacific storms are tracked days ahead and you should be able to track them on TV news programs. You should be willing to hole up if one is about to hit you.
Suggestion for parks:
1. Death Valley -- it is at its high season over the Xmas holidays. It could well be very crowded. Temperatures are usually quite mild and the scenery is spectacular. Your elevation in the camps will be around sea level or below.
2. Grand Canyon, South Rim -- This is possibly do-able but you should expect snow on the ground and very cold nights. Stay at a full hook up park, or in a hotel at the Village. Make local inquiry before going. The Canyon in winter is spectacular but your elevation will be at some 6800 feet.
3. Anza Borrego Desert State Park, Borrego Springs, CA -- A huge California state park with 625,000 acres of very accessible desert. The altitude will be aound 1000 feet and you can expect it to be about 25ºF warmer than the Grand Canyon, just from the effect of altitude alone. Nights will be quite cool tho. Rental Jeeps and guided 4WD tours are available. The facilities are small-town American desert town. The scenery is spectacular but not as much as the previous two parks which are world class.
4. Tucson, AZ -- not a park but rather a city with loads of parks and sightseeing in and around it. The altitude is around 2400 feet and the climate is relatively mild in the winter. Xmas is nice season in the town. They share in the Mexican celebrations of Navidad which tend to be colorful and cheerful.
5. San Diego, CA -- possessed of the finest climate in the USA, bar Hawaii. SD is a marvelous tourist town: beaches, mountains, Old Town, Mission Bay, Coronado, Scripps Acquarium, Balboa Park and its museums, the great San Diego Zoo and Wild Animal Park, the US Navy, fishing, hiking, etc. etc. Lots of RV parks.
With 8 days, consider a loop around LV - Death Valley or Anza Borrego - San Diego - LV. Another possiblity might be LV - Grand Canyon - Tucson - LV.