Coat slow cooker bowl with nonstick cooking spray.
Insert slicing disk into food processor and thinly slice zucchini (or slice by hand). Transfer to a large bowl and add mushrooms and sweet pepper. In medium-size bowl, combine ricotta, 1 cup shredded cheese, parsley, garlic and black pepper.
Spread 1/2 cup pasta sauce over bottom of slow cooker. Top with 2 noodles (4 halves), overlapping as needed. Blend 3/4 cup pasta sauce with 1/2 cup water and set aside. Stir remaining sauce and 2 tablespoons of the Parmesan into zucchini mixture. Layer half of the zucchini mixture over noodles. Top with 2 more noodles (4 halves). Spread with ricotta mixture and top with 2 more noodles. Finish layering with remaining zucchini mixture and remaining 2 noodles. Spread with reserved sauce-water mixture.
Cover and cook on HIGH for 4 1/2 hours or on LOW for 6 1/2 hours, adding remaining 1 cup shredded cheese and 2 tablespoons grated Parmesan for last 10 minutes of cook time. Let stand 10 minutes before serving.