I think that you need to give more information. How close to Ft Myers do you need to be? What kind and length of RV do you have, what kind of activities are you looking for, how close to the beach, do you require FHUs?
There is state park in Estero, which is 15 miles away, Koreshan, which has a highly reviewed campground at $26.00 a night, but it is only electric and water, no sewer, only can accomodate up to 40 foot, and doesn't have much in the way of activities or amenities.
Be careful with "cheap" or "affordable" because really, in that area, you wont get much.