I'll just pick up on the suggestion to stop by Fredericksburg. There are several good choices of campgrounds. We usually stay at the KOA just east of the city. This is one KOA I can recommend. Obviously we prefer it to any other campground in Fredericksburg.
If you are coming down I-10 from Casa Grande, you would turn east on US 290 to take you straight into Fredericksburg. We will be going there next month, though we may be a bit late for the bluebonnets.
You can then drop down quite easily to San Antonio, either through Comfort, TX or Boerne, TX--the latter will take you through Luckenbach.
Just off US 281 south of San Antonio, we often stay at Choke Canyon State Park, Calliham Unit. On a lake with wildlife often seen in the campground.
And that would put you in Donna the next day with an easy trip.