Last February 1st my wife and I left Michigan for our first snowbird trip to Florida. We spent 6 weeks down there and while we really didn't make reservations per se we did end up finding out about someone on this site that was cancelling their state park reservations at Oscar Sherer SP in Osprey. We ended up reserving 9 days there. FYI: State parks in the lower half of Florida are usually full throughout the winter months, although you may be able to get into Highland Hammocks SP in Sebring. Being away from the coasts makes for easier drive up camping.
Basically we didn't make reservations and ended up camping at 9 different parks. More than half the time we did call ahead to reserve a site at places we wanted to make sure of before traveling there. I did this 1-3 days ahead. I created a list of about 35 potential RV parks before leaving Michigan. I used RV Park Review.com to research different parks and listed contact numbers, websites, rates, amenities, comments from other campers, etc. This way I narrowed down parks that we more than likely preferred due to location, amenities, or cost. While reservations are not always necessary you really need to bone up on the possibilities, especially if it's your first trip down there. You don't want to be without a site at 7 p.m. unless you can find a friendly Walmart.
We stayed at most parks for about 3-4 days. They always had space for us and there were always vacant sites. One time we extended our stay from 4 days to 7 because we liked the park. There may be some popular parks, especially those closest to the coasts in high tourist areas, that fill up to the point you can't get in. We just never found that ourselves, but you will have several other posters here tell you that reservations are necessary. They are not necessary because there are many thousands of campsites throughout Florida. Since you're willing to be flexible you shouldn't have a problem. But do your homework first.
