Blue Springs State PK in Orange City, FL (just south of Deland) is one of the best places to view manatees. During winter months manatees flock to the springs because they are warmer than the St. Johns River water. Not unusual to see dozens of manatees in the spring and run. Has RV camping, with electric & water, rigs limited to 40 ft or less. Also have cabins and a lodge. Very popular and busy over winter months so reservations are advised.
https://www.floridastateparks.org/park/Blue-Spring