In a large skillet cook sausage and mushrooms in hot oil over medium heat until meat is browned, stirring to break up meat as it cooks. Drain off fat, if needed.
Coat the inside of a 3½- to 4-quart slow cooker with cooking spray. Add the cooked sausage mixture, drained tomatoes, onion, sweet pepper, pepperoni, tomato sauce, oregano, fennel, salt and garlic powder. Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours.
Cook pasta according to package directions. Drain. Add cooked pasta to slow cooker. Stir to combine. If desired, top with Parmesan cheese.