Pierce the beef roast to tenderize it and let the flavors in, and place it into a slow cooker with the garlic, red bell peppers, green bell peppers, yellow onions and sweet onions. Stir in the tomato paste and red wine vinegar. Sprinkle with the garlic powder, seasoned salt, salt, and ground black pepper.
Cover, and cook on Low until the beef is tender, 7 to 8 hours. Remove beef, and shred into long strands using two forks. Return the meat to the slow cooker and combine with the vegetables.
Cook on Low until the meat and vegetables are very tender, about 30 minutes.