If you can afford to go stay at the keys, why do you need a free spot? I don't feel that I can afford a place like the keys but I always pay for a camp site no matter where we go. The exception to that is if I can't find any place open in winter, in that case, we did stay at a flying J twice 10 years ago because no place else was open.
I'm going to the Keys because I found a special promotion for $400.00 for the month, otherwise I could not afford the keys. I do not stay at campgrounds when we travel because they are usually far from the interstate, tight sites that are difficult to get into and too expensive for simply pulling in at the end of the day and leaving by 7:00 am the next morning. I much rather stay at a Flying J