Any place you get to in the Tampa area will have good weather, and lots to see and do. We always stayed at Robert's RV and Mobile Home park in St Petes: http://www.robertsrv.com/ it provides basic sites, with a pool, it is clean, and lots of in park activities, but it isn't high end. Walking distance to Walmart, Target and several restaurants, and about 15 minutes to the beach.
As for gas, we are about 300 kms further from where we keep the coach in Moncton/Shediac, but I gave up tracking fuel costs as it doesn't really matter. We go where we want to go, fill up the tank at the station showing the best price at the time, and drive for another day or so.
As for heading down, we try to head down the I 81, via I 86 or I 84, or the 401 to the 81, the to Charlotte on the I 77, to the I 95, over to Ocala on the 301 to the I 75.
You'll have a great trip.
Ed