Slow Cooker Pizza Casserole

1 1/2 lbs ground beef , cooked and drained

1 lb sausage , cooked and drained

1 lb corkscrew macaroni , cooked and drained

2 cups shedded mozzarella cheese

2 cups freshly grated parmesan cheese

2 (4 ounce) cans mushroom stems and pieces , drained

2 (3 ounce) packages sliced pepperoni

1/2 cup finely chopped onion

1/2 cup finely chopped green bell pepper

1 garlic clove , minced