You might wanna check out this map. Hopefully that link will load the map with Enchanted Rock in the dead center. Enchanted Rock is in that dark blue band, so it has some light pollution, but very little. I've never personally seen the Milky Way from E Rock, but I've only been up there in the middle of the night on a couple of occasions, both fairly cloudy. If you do a Google image search for "Enchanted Rock Milky Way," you get some good results, but just because a camera captured it doesn't mean that the naked eye will, at least not to that degree.

The closest absolute "dark sky" from Austin or San Antonio is gonna be Big Bend. Definitely worth the trip if you really want to see the Milky Way!