I did some research before our trip to San Antonio and reached out to some friends that are San Antonio natives. One friend who grew up in San Antonio said that Rolando's Super Taco is the "most authentic place in San Antonio" and we had to agree!
The restaurant is located off the beaten path but not too far (10min drive from downtown). The tacos are huge so go there hungry. And for the full experience order the "Nando Taco" consisting of carne guisada, cheese, bacon, and beans. And don't forget to get it "con papas"! Enjoy!
They close at 3pm most days so only breakfast and lunch options are available. But look at it this way... if you eat a super taco for lunch you can probably skip dinner.