I had to look up Shark Valley, been a long time since I was out that way, we always called it loop road, a bumpy dirt road. Anyway, there's a lot of suggestions for your "2-3 days". If you do get down to Key West there is plenty to do if you had more time and like history stuff, Ft. Jefferson (Dry Tortugas) is something to see. Also on your way down, as you are traveling over the new 7 mile bridge, take a good look at the old one. Imagine yourself on that bridge with your camper and meeting a semi going the opposite way.