I'll second the Cracker Barrel suggestion. Shoney's would also be a good second choice. Several Cracker Barrel's come to mind: west of Knoxville where I-75/I-40 run together (two locations; Cedar Bluff Road being one), and just south of the I-75/I-40 split near Sweetwater. Shoney's will also be in west Knoxville (Walker Springs Road). The west Knoxville locations should be near halfway through Tennessee for you.
You have fewer choices between Knoxville and Cleveland (north of Chattanooga), so you may wish to stop before then. The only thing I'm sure of north of Knoxville is Perkins, though there are other choices.