In a large skillet, over medium-high heat, crumble and cook Italian sausage until browned; drain and add to slow cooker.
Add marinara sauce, mushrooms, and diced tomatoes to slow cooker.
Cover, and cook on low 7-8 hours or on high 3-4 hours.
Stir in tortellini, then sprinkle with cheese.
Cover, and cook 10-15 minutes more on low, or until tortellini is cooked through.