Jim,
You can't avoid Houston so try to hit it in non-rush hour time slots. San Antonio is similar but you can avoid it by exiting at Seguin on to TX 46 and take it over to Boerne TX on I10 NW of San Antonio. Road is 2 lane but good. Agree re Buckhorn in Kerrville. If it's full, Guadalupe River RV Park is also very good. Just head to Kerrville on Goat Creek Rd (same rd Buckhorn is on) and turn right at main drag. Nice place. Also, Billie Jeans is good place for grub.
Enjoy. Steve