We have ventured to Florida two winters now, and will be making number three in a few weeks. A few things we have learned: don't be there Jan, Feb, and March unless you have a cheap place to stay, don't get there much before October unless you dig humidity, and there are a lot of old people there. :)
The first three months of the year are crowded and expensive in south Florida. If you want to stay above the arctic circle (Ocala), there are deals to be had, and much fewer people. The southerners think it is too cold above Ocala.
Up in the northern-Gulf side, there is a great little park to stop at on the way down. It is called Natures Coast RV Resort
in Steinhatchee, FL. It has like 15 spaces, surrounded by nature, close drive to a funky little fishing town, great owners. Worth a night, week, or month.
We have done both years with minimal reservations. We made reservations for Ft Desoto because we really wanted to stay there, and it is a popular place.
As for rates, May thru October is the cheapest, Feb and March typically the most expensive. For example, we stayed at Sun N Fun in Sarasota a few times. A week in October is ~$225, a month $840. In February, that goes to about $450 a week, $1800 for the month.
Hit us up - we will be down by Ft. Meyers in Nov and Dec.