Re FL and reservations....strongly suggest you take care of that before you leave home. Most everywhere is pretty full and what remains may not be what you'd like. Agree on Kissimmee....very nice, away from the main crowds. Believe it was a KOA we stayed at several years ago. Not normally a KOA fan, but this was very nice and good size sites.
Back to Lakeland....from Jackson, TN to Lakeland is 792 miles. If you work it just right, you can overnight at the CW in Calera, AL - just south of Birmingham - with FHU at no charge if space available. Shopping is appreciated and I just always plan to get our black tank treatment + other necessities. The next stop, if needed is the CW Tallahassee. You may get FHU or just water/electric, but again no charge. Our DS has to have numerous stops to get out and move around, so it takes us 3 travel days when towing. There are not a huge number of CGs along I-65 and we usually take advantage of their hospitality.