Heat oven to 400. Place chicken breast on baking sheets. Lightly coat with olive oil and evenly sprinkle with salt and pepper. If thawed, bake for 20 to 30 minutes. If frozen, bake for 25 to 35 minutes. Remove from oven and shredded or cut into thin chicken strips.
Meanwhile in a large slow cooker, combine enchilada sauce, tomatoes, jalapenos, cilantro, garlic, chili powder, cumin, salt, and pepper. Stir well.
Add onion, squash, zucchini, corn, and baked chicken. Stir well.
Cook 2 to 3 hours on HIGH or 3 to 4 hours on LOW.