We pulled our 5th wheel down to MM47 at Marathon last winter. Stayed at Knight's Key RV Park (about $70/night for water & elec.). This is a laid back campground with plenty of room at most sites. Some close to the canal were tight though. You can get down to Key West within a hour. While the prices are high down in the Keys you don't have to stay forever. We stayed just 4 days and had to get away as Key West was too much fun.
Just up the highway from the Keys is Miami Everglades RV Resort. It's less expensive than the Keys and you're not far from them if you want to drive down there. It's about 15 or so miles from Miami out in the farmland north of Homestead. Went to South Beach one day. That was fun!