Your best bet in the FT Myers area is Gulf Waters RV Park. It's where we stay when we go "home" to visit. It's near the bridge to Sanibel, as well as the turn off to the beach.
Near Sanibel, check out Red Coconut Rv Resort located in Ft Myers Beach (http://redcoconut.com/). In Miami, on the way to the keys, I enjoyed Larry and Penny Thompson Park, which is managed by the county (http://www.miamidade.gov/parks/Parks/larry_penny.asp).