Stir tomatoes with cream, milk and garlic in a large bowl.
Line bottom and sides of slow cooker with aluminum foil. Spread a quarter of tomato mixture over bottom. Crumble a third of sausage meat over sauce, then a third of mushrooms. Arrange a third of ravioli in an even layer over mushrooms. Repeat with remaining tomato sauce, sausage, mushrooms and ravioli, ending with sauce. Sprinkle evenly with cheese.
Cover and cook on low, 6 to 8 hours. Remove lid and let stand for 10 minutes.
Serve immediately or freeze for a later meal!