I just used National, which is in the same spot as Alamo, over Thanksgiving.
When you exit baggage claim, you head off to the right, where there is a little U shaped driveway marked shuttles. The Alamo/National shuttle circles between there and their kiosk continuously. We waited for less than one minute for the shuttle. The driver will ask if you are National or Alamo, and then take you over to the kiosks, which are on airport property, but over on a side road. It's less than a 3 minute ride.