Arrange onion, potatoes, carrots, turnip, and rutabaga in bottom of slow cooker. Season beef all over with salt and black pepper. Rub flour all over beef. Place roast on top of vegetables in slow cooker.
Whisk together tomato sauce, brown sugar, chili powder, cumin, mustard powder, and garlic powder. Pour mixture over beef. Cover and cook on LOW for 12 hours or on HIGH for 8 hours.
Serve 1/3 of beef and all of the vegetables with this meal. Shred and refrigerate remaining beef until ready to use.