I have tried for the last 2 years to get a reservation for any Florida Keys State Park. Impossible! It is unfortunate to have to try and manage daily viewing of any availability. It not just the Keys. Any of the beach related Florida state parks are the same. Last month I spent the day at Bahia Honda. Always beautiful but having spent 2 nights there 2 years ago I guess I was just lucky. I know most states want to keep their state park fees low but they aren't charging enough to maintain them. They are still messing with Long Key rv spots. Their is no secret to the now numerous camp site search engines. They make money for someone but less than useful for getting a site in a high demand park. And above all forget traveling with friends to these places. I sure won't post further great times for fear of attracting attention for the spots I am dreaming about. That is my 2 cents worth of advice.
It doesn't help that the reservation system gives Florida residents a 30 day head start on locking down the most desireable parks before anyone else gets a shot at them.