A few in N. Florida.
You can count the beachfront campgrounds pretty much on one hand.
Beverly Beach Camptown is $100+ a night.
North Beach near St Augustine is close enough to walk to the beach and is on the Intracoastal Waterway.
It will be pretty to look at but water will be chilly in the 60s. We swam in Superior in MI 55 in July.
I would recommend Ocala area for a month and visit all the springs. Great campgrounds cheap. Try Grand Lake RV Resort in Citra.
Third area would be a coin toss between St Pete/ Sarasota or South Florida.
Check out John Prince near WPB.
Check out Larry and Penney Thompson near Miami.
Both Excellent county parks that take snow birds.
State Parks only do two weeks and are hard to get into. You can book them 11 months out.