Sauté first 3 ingredients in a large nonstick skillet coated with cooking spray over medium-high heat 10 minutes or until onions are translucent. Add mushrooms, zucchini, and squash; sauté 3 more minutes. Add chili powder and next 2 ingredients, and sauté 5 more minutes.
Stir together tomato sauce and tomato juice in a 6-qt. slow cooker until smooth. Stir in diced tomatoes, next 2 ingredients, and carrot mixture. Cover and cook on LOW 8 hours.