The keys

A couple of years ago we stayed in Marathon at Knight's Key RV Resort. Basic campground with a nice tiki bar to watch sunsets. It's at Mile Marker 47. Reasonably priced. No sewer, but they will pump you out at your site. We like to walk in the mornings and you're right at the 7 Mile Bridge with the old bridge to walk/bike on for several miles. There are others in this area. Jolly Roger appears to be favored by many.

Here's a RV Park Reviews.com for Marathon campgrounds to check out.

http://www.rvparkreviews.com/regions/florida/marathon