Of Course I-24 and I-40 are the quickest route to Memphis (and 840 is now open all the way around from Murfreesboro to Dickson.
When you get to West Memphis, I recommend taking 55 north to Marion and follow US-64 west to Conway.
It is mostly two-lane road, but it is a far more pleasant drive that I-40 between Memphis and Little Rock.
You can pick up I-40 at Conway to OK City then I-44 to Ft Sill
If you want a more scenic route than I-40 through Oklahoma, at Little Rock, take I-30 West to the Hot Springs Exit. Hot Springs is a fun little town, with, among other things, some nice hot Springs
Would be a nice stopover point.
Then from Hot Springs, take US-70 westward. Again, a lot of two-lane small towns, and some hills. But you see some of the nicest country Oklahoma has to offer and IMHO, it beats fretting through I-40 traffic for a whole day.
The scenic routes will cost you a couple, three extra hours driving time.
If you decide to stay with I-40, Russellville, AR is a good stopover point, with Park service campgrounds, and some decent RV parks, and decent restaurants.
