California will be very expensive. FL and AZ less so and they are welcoming to snowbirds with many campgrounds targeteted at these camper. Az parks are mainly south of Phoenix. Fl has parks state wide but below Orlando they get a LOT more expensive especially close to either shore.
You may have a difficult time finding spots in January as you will be late to the game.
Florida is one of the three preferred domicile states since it has no income tax, TX and SD are the others.
I know nothing about renting RV's.