I run from Fort Worth to Bristol TN and Asheville NC once a year in my 36ft HR Endeavor. I have done both the North and South routes several times and prefer I-40 and I-30. Maybe a few miles further but I like the scenery better. Just returned 3 weeks ago and the both interstates are good with very little construction.
As Golfcart said the I-20 route can be boring. Done that one a couple of times but have been using the I-40/I-30 route for the last 5-6 years.
Let me know if you need any campground info along 40/30.