If you're in no big rush drive west on I-20 from DFW & take 281 south to SA. Or 820 west to 377 then SW to 281 south. It's a much nicer drive & you avoid Austin.
If you do take the I-35 be sure when needing fuel/food to stop at Buc Ees, they probably the nicest, cleanest convenience store/fuel stations you'll ever be in & ALL of them are VERY rv friendly for fuel & parking.
No I don't work there or get any compensation from them, just a great place to stop.