Have driven Memphis to Amarillo (720 miles) 10-12 times over the last 35 years in a truck (NOT towing). About 12 hours with 1 fuel stop at posted speeds. One suggestion, consider taking US-64 across Arkansas from Marion (just north of West Memphis) to Conway - about 1/2 of it is either 4 lane or 3 lane (alternate passing lane every 5-10 miles on the eastern end), much less truck traffic - rejoin I-40 at Conway. This also avoids Little Rock. Also suggest taking I-40 straight through Oklahoma City. Memphis-Little Rock truck traffic is very heavy - 1/2 or more of it takes I-30 out of Little Rock west to Texas while Conway to Oklahoma is less truck traffic.
