I don't think you are going to be able to get too far South, enjoy some warmth and get again during a week long trip. The other issue is you want to go were everyone else wants to go at the same time they want to go there so finding somewhere to stay could be challenging. That really applies to Spring break warm spots.
As to the RV, it really all about the floor plan. Find something you think you love, spend a few hours in it thr way you think you would live in it, drive it a reasonable distance to see if you like it and can handle it. No need to go diesel with your limited travel plans but if you find one you love, that's even better.
