You're starting from the same place we do when pulling our unit down to FL for the winters. We'll be coming back on you're way down. Here's our route, and we don't worry about Nashville. Just stay in your lane and continue south.
Down 131 to Middlebury, IN. Take 20 west to the south side of South Bend (this is actually shorter than I-69 by about 30 miles) and then take US31 to Indy. Then I-465 to I-65 to Louisville. Stay on 65 thru Nashville and on to Montgomery, AL. In Montgomery take I-85 a very short distance east to exit #6 (can't think of the road name) which you follow maybe 4-5 miles to US-231 south to I-10 in the FL panhandle. Take I-10 east to exit #225 & south on US19/27. This 19/27 route is 4-lane and lightly traveled. If you're going further south you should take US98 south of Homosassa Springs over thru Brooksville (all 4-lane) and hook up to I-75.
This route will keep you off I-75 until you get into central FL. It's relatively flat compared to 75, you avoid Cincinnati & Atlanta and all that spring traffic heading south on 75. Granted, there will be traffic, but not as bad as 75. It's about 1200 miles to Crystal River, FL (where we winter) from Portage. We're in Crystal River late the 3rd day on the road. One night in a motel & one night at Deer Run RV Park about 5-6 miles north of Troy, AL & right along US231. From there it's 350 miles to Crystal River.