Isaac - your are correct for that area. We went through there in the spring and had no problems getting a site on short notice. It's a different story in the more populous areas of the country. Cape Cod, June through September, you need reservations, weekend and week day.
South of I-4 in Florida (considered to be the >70 degrees line), people make reservations for next winter when they leave in the spring. I've seen several posts here announcing an upcoming cancellation in the Keys that get snapped up immediately.
Just a few years ago when I was researching being on the road for a winter, the general consensus was that unless you were going to extreme south Florida, reservations weren't needed. I found that mostly to be true, but we had reservations in advance for south of I-4.
And as I said, if you want to pay $100 a night, you can usually get a site anywhere. We tried to average $35 a night. One night in the Keys would have meant 3 nights in a Walmart :)