Use a 6-quart slow cooker. Spread a length of aluminum foil onto the counter. Place the butterflied meat in the middle of the foil. Sprinkle with celery seed, garlic powder, and salt, and pour liquid smoke onto the middle of the meat. Fold the meat together, and gather the edges of the foil to create a tight packet. Place into the slow cooker.

Wrap each of the sweet potato halves in foil and nestle each packet around the roast. Do not add water. Cover and cook on low for 8 hours. Carefully remove the packets from the pot and open away from yourself, as the steam will be hot.