You might want to list some of your preferences. Further north along the Nature Coast (Cedar Key, Crystal River, etc.) or further south like Sarasota & Naples? Close to the ocean/beaches or a bit further inland? Temps certainly vary north to south as you probably know.
We enjoy the Nature Coast area around Crystal River. It's not as crowded as areas south of Tampa, but it's not so rural that you can't find the usual chain stores. Kayaking and fishing are popular and the seafood is great. The weather is a bit cooler in January and early February, but it warms up into the 60's and low 70's regularly. Most of March was regularly in the 80's with cooler nights.
We've also stayed down in the Port Charlotte/Punta Gorda area, but inland a bit. Riverside RV Park (Arcadia) is nice and while the address is Arcadia it's only about 4-5 miles east of I-75.
It's getting busier and reservations are becoming tighter so it's wise to do your research now so you can jump on a reservation for next season. Our park, Rock Crusher Canyon RV Park, saw more people making reservations for next season when they had open sign up in time in late February.