On the 2017 Supercharger possibilities map, there is a marker for a San Antonio location.
The likely location might be at the new Service Center that is permitted at 23011 I-10 W. The permits so far for this location do not show any mention of a Supercharger.
Taking a broader view, the routes to be served by a San Antonio location would be I-10 East and West, I-35 North and South, and I-37 South. Placement in the Service Center would be ideal for the I-10 traffic, but very inconvenient for the other routes.
A better location might be on I-10, somewhere between the intersection with I-35 and I-37. This would be usable for all three routes.
To Junction, TX - 121 mi
To Flatonia, TX - 89 mi
To San Marcos, TX - 47 mi
To Pearsall, TX - 59 mi
To Three Rivers, TX - 69 mi
My money is still on the Service Center location, even though it is not ideal for anything but I-10. I'm going off the precedent set by the Supercharger at the Houston Service Center, which isn't real convenient for travel in most directions, yet there it is.