In South Texas get on Hwy 281 north.
Hwy 281 N to I-37 N take (exit 133) and get on I-410 N
I 410N to I 35N
I 35 N to Hwy 46W (exit 190)
Hwy 46 to Hwy 281N
Follow 281 N to a few miles south of Jacksboro TX (380)
Take 380 east to Hwy 51- Turn left (north)
Follow Hwy 51N to Gainesville and I 35 N.
The main purpose of these directions is the avoidance of the Austin to Waco section of I 35. If you do it once, especially in an RV you will have a firm understanding of why you need to avoid this section.