We have traveled from Tucson to San Antonio many times, but we were always in a hurry and didn't have any time for sightseeing. From San Antonio, we took Interstate 10 all the way. Spent the night at the KOA in Van Horn, TX, both coming and going. Fuel stops in Tucson, Las Cruces, Van Horn and San Antonio. We always take two days for this leg of our trips.
That leg is about 870 miles and I like to drive 450 miles per day or less. It's an easy drive, multi-lane interstate all the way, scenic if you like desert. But the drive through the Texas Hill Country is really nice.