Google maps put the drive from Detroit to Valdosta at 954 mi (about 16 hours 55 mins). Knoxville, TN, is about halfway, at 512 mi (about 8 hours 55 mins) south of Detroit. Another hour or so and they would be in Chattanooga.
Not sure about hotel recommendations for Knoxville. I think there's a fairly nice area west of the city off I-75 but it's been a while since I've been through there so I'm not sure. Chattanooga area I would recommend the Hamilton Mall area east of the city, exit 5, IIRC.
Another suggestion ... Google's directions to Valdosta recommended taking I-285 around Atlanta. Since there are two people, take the HOV lanes on I-75 straight through the city. Traffic on the Perimeter can be as bad as traffic in the city itself.