We are going to be in the SW Florida area the end of May and would like feed back re which would be better -Sanibel Island or Siesta Key. We will be camping with an RV and are interested in outdoor activities, biking, fishing and nice beaches. Any input would be appreciated.

The only thing I could tell you about Sanibel, is that it's a beautiful place, but I have no knowledge of camping facilities. On the other hand, since I live on Siesta Key, I could tell you few things. The only campground is located at the south end of the key. I believe it has about 50 spots and is located directly on the beach. You will be able to fish, enjoy the beach without crossing any street. Also there is a marina almost directly on the other side of the street, where you could rent or charter boats. Biking is done on the road leading to the campground, and since it's mostly residential, there is not much traffic. Although there are couple of restaurants in the area, the closest 'real' commercial area is about 2.5 miles away at the south bridge. Restaurants, food market and some shops. The Siesta Village(where I live) is another 2.5 miles from there. More of a tourist paradise, it has some fun restaurants, few night spots where locals and visitors mix, and many more shops. In the evening it's great people watching as you sip a daiquari on one of the bar decks. All our beaches are public, so it makes them great walking beaches, as well as Siesta is known to have some of the finest purest sand around.
All the roads have designated bike trails here on the island, so you could bike the whole island.
I will tell you that if you like nature, Myakka State Park is worth a day trip. You could bike, rent a canoe and just enjoy the wildlife, alligators, variety of birds, occasionally a Florida panther or 2, although I've never seen one. It's a nature reserve, so if you enjoy such things, you will have fun. There is also a campground there as well.
Have a great trip!