The "best" parks in that area in my opinion are Broward and Palm Beach County parks. Good facilities, reasonable rates. A great many of the parks in that area only rent by the month that time of year. The county parks have daily rates and spots.
If the park is booked at first try, call the park again in a week. We did that for four weeks before we got a spot at TY Park.
The US Army Corps of Engineers has s nice small park next to the lock for the cross Florida canal at Port St Lucie. But it is hard to get a reservation.
We've also stayed at The Savannas - a St Lucie County park near Fort Pierce. Very nice.
Search for the county name and parks. I.e. Palm Beach County Parks for more info.