I've driven cross-country a couple of times for work (DC to Los Angeles) and usually do it in about five days.

The drive through Arkansas can be a real pain in the butt, and I mean that seriously. If the transportation department has done their job, they will have fixed all the badly filled cracks in the asphalt. At 65 mph or so, the vibrations can be numbing. Haven't driven the route in a couple of years, and back then they were doing construction, so maybe they fixed it.

Not much to see in OKC, but a stay there wouldn't be bad. OKC to Albuquerque is nothing special, but I think you gain an hour between TX and NM. Plus, there's usually a strong headwind going through NM, so depending on the profile of his vehicle, your dad may or may not notice he'll have to put in a little more input into the steering wheel.

I like staying in the larger cities because it gives more hotel options and places to find something to eat. Plus a larger supermarket to get snacks for the road - the gas stations never have anything relatively healthy.

How about:

1. Knoxville - Memphis
2. Memphis - OKC
3. OKC - Albuquerque
4. Albuquerque - Phoenix

The OKC - Albuquerque stretch may the long route, so I see your point about getting through OKC. You may want to try Weatherford, Clinton, or Elk City (Elk City being closest to the TX border). I think there should be Holiday Inns at these smaller cities.

Good luck!