Make a reservation online in advance (as far in advance as you are certain you'll be there) at https://public.co.pinellas.fl.us/parks/ParksMain.jsp
Although, good luck. These sites are almost always booked up months in advance (I mean months) since Pinellas County residents can book 7 months in advance, but they are changing up the reservation and cancellation fees and process up which will hopefully make it easier to get a site. (I was able to stay with a friend whom is a resident).
The waterfront sites are awesome, especially if you have a kayak or canoe. I personally like the tent, popup, van only sites more (we didn't have an RV and I don't like the noise they generate). Instead of heading to the main beach areas is you can get the even numbered sites 30-40 where the beach meet the mangroves you’ll have a much better time as you won't have many people around and right out in front of your site you’ll have great beach access. All the sites are all nicely tucked away under the trees providing great shade and excellent flat spots.
All of the sites have water and electrical hook-up (30-50 amp service), an like normal picnic table and a grill. The campground also has a small camp store, dump stations and pretty nice modern restrooms with showers and laundry facilities.
The Park itself is awesome, great beaches, the fort is really cool. There are miles of nice paved paths for biking around and really just lots of space.