Rub with onion soup mix, salt and pepper. Place pork butt into slow cooker. Top with root beer and 1/2 cup of BBQ sauce. Cover and cook on low for 10 hours or until pork is very tender, falls apart and shreds easily.
Remove meat from slow cooker. Pour out and discard all but 3/4 cup juices. Use 2 forks to shred meat, discarding any extra fatty pieces.
Place meat back into slow cooker with reserved juices. Add remaining BBQ sauce and stir to combine. Cover and heat through 15-20 minutes more. Serve on warmed hamburger buns with coleslaw, if desired.