San Diego to Texas in December

I second the Big Bend National Park, the park is really big. It has one full hookup rv place in the part(if my memory is correct), it is Rio grande Village RV. The park also has alot of no hookup sites. Carlsbad, Davis Mountains, are also good. If you want the ocean, I would also recommend staying south near S. Padre.
We also, always have fun going to San Antonio for a few days. It really depends on what you want to do.