Place the pork tenderloin in a slow cooker. Pour the root beer over the meat. Cover and cook on low until well cooked and shreds easily, approx. 6 hours. The actual time will vary with individual slow cooker.
Take meat out of cooker and shred with fork (it should easily fall apart). Stir in barbecue sauce or let people add their own sauce on bun.
Serve on top of buttered and toasted buns. Slaw topping is optional.