Spud, to some degree it may well depend on where geographically you will stay "north of I4". I think if you stay closer to the Georgia line or inland away from the coasts, you will have better luck finding a spot without reservations. If you want to stay closer to Orlando or near the coasts, you will likely have a more difficult time.
Since this will be your first trip to Florida, some would suggest staying a one place a couple of weeks, then moving on to explore another place, is a good strategy. This could help you find a special place where you might want to spend the season on future trips.
Be advised, if you stay north of I4, you may run into some below freezing temps. at times. Most believe if you want to stay away from freezing temps, you need to be south of I4, which is in general, a more expensive trip. We have experienced below freezing temps in the Orlando area.
Best wishes for a safe trip.