I would suggest heading north from Naples on I-75 to the US-98 exit and then west through Brooksville to US-19 north. Once you're north of Crystal River it's a nice clear shot up to I-10. Take I-10 west to US-231 and head north to Montgomery, AL. (There is a better route once you hit Montgomery by taking a right and then up a short distance to I-85 and then back west to I-65, but I can't remember the road name. It's the way I use though.) US-231 is a nice 4 lane road. It's a bit slow going around Dothan, AL using the by pass, but not so bad as Atlanta or Cincinnati by a long shot.
Take I-65 as far north as you want. Since we go to SW Michigan we take I-69 north from Indianapolis.
BTW, there's a nice campground right along US-231 north of Troy, AL a few miles called Deer Run RV Park. It's right on the west side of 231. It's a FHU park with cable and if you have a Good Sam card it's about $27/night.
