Start with taking US 19/98 north to Albany then US 520 toward Columbus. It meets with US 280 at Richland. North to LaGrange, and follow US 27
That can take you to and through Chattanooga.
Also, US 27 is pretty good road all the way from Chattanooga to Lexington/Frankfort, KY area.
A friend/CoWorker from Indiana, told me that From Frankfort, US 421 will get you across the Ohio river and then SR 3 is a nice run from Greensburg to Ft Wayne. But, I haven't qualified that for an RV/High Clearance route, myself, yet.