Spray slow cooker with nonstick spray. Add beef roast. (No extra liquid is needed. The roast will simmer in its own juices.) Cover and cook on low for 8 hours. Drain the fat and shred the beef. Return shredded beef to the slow cooker.
In a medium bowl, combine the remaining ingredients. Add sauce to the shredded beef and stir to combine. Cover and cook on low for 4 hours. (See "Recipe Notes" about preparation below.)
My sister usually cooks the beef in the slow cooker overnight. Then, in the morning, she drains the fat, shreds the meat, adds the sauce, and continues cooking the barbecue beef in the slow cooker until lunch time.