To get to Murfreesboro (Crater of Diamonds SP) the easiest way is to take 71 north at Texarkana (State Line Ave) You will go about 31 miles and turn left onto AR 27. This will take you through Nashville to Murfreesboro.
When you're ready to leave there to head to Clarksville you can continue north on 27 and get on HWY 70 in Kirby and it will take you to Hot Springs. From here you can continue on 70 and will merge with I-30 again. Hwy 7 out of Hot Springs is a beautiful drive but is very mountainous and curvy. It looks like a good shortcut but if you're not familiar with driving a large RV don't take it. Just head to Little Rock and then take I-40 to Clarksville. Petit Jean SP and Mt Magazine are on this route if you have time to stop either place.
You can also take 19 out of Murfreesboro toward Prescott and hit I-30 more directly but I haven't been on that road for many years so don't know what kind of shape it is in..
Correction: Turn Right on 27 off of 71 (not left) There is no left turn anyway!!! LOL
Have fun and enjoy that kiddo!