Cook and crumble sausage. Saute the peppers, onions, and mushrooms in the grease. You can just put these straight into the slow cooker without sauteing if you are short on time.
Place all ingredients into slow cooker, except for the mozzarella cheese. Cook on low for 2-3 hours, or until heated through.
30 minutes before serving, sprinkle with mozzarella cheese. If your slow cooker pot can go in the oven, broil the cheese before serving.
*Alternatively, you can leave the pepperonis out of the mixture, and place them on top for the last hour of cooking for more of a "pizza" look.