Lightly coat the inside of a 5-1/2- or 6-quart slow cooker with cooking spray. Pour 1 cup of the spaghetti sauce into the prepared cooker. Add frozen ravioli from one package and the meatballs. Sprinkle with 1 cup of the mozzarella cheese. Top with the remaining spaghetti sauce from first jar. Add ravioli from the remaining package and the remaining 1 cup mozzarella cheese. Pour spaghetti sauce from second jar over mixture in cooker.
Cover and cook on low-heat setting for 4-1/2 to 5 hours or on high-heat setting for 2-1/2 to 3 hours.
Remove liner from cooker, if possible, or turn off cooker. Sprinkle ravioli mixture with Parmesan cheese. Cover and let stand for 15 minutes before serving.