Hi Dave! Hope you like heat, because it is going to be HOT and HUMID in the Keys in a few months. Like New Orleans in the summer, if you have ever been there. It's ghastly, in our opinion, but some folks like that kind of weather.
The drive down from Miami through the keys is worth the trip, though, to-die-for beautiful.
We have been to Key West a couple of times, stayed at Boyd's RV park last year. It is very nice, very expensive. They have a website. There are a number of State parks down in there, right on the water, although we have never stayed in them. I don't think there is anything cheap in the Keys. If you are looking for a site, I would suggest making reservations.
Have a good time, but bring lots of sunblock.