Microwave broth, cream cheese, cornstarch, chipotle, garlic, and pepper in large bowl, whisking occasionally, until smooth and thickened, about 5 minutes. Stir in Monterey Jack and American cheeses until combined.
For a 1 1/2- to 5-quart slow cooker: Transfer mixture to slow cooker, cover, and cook until cheese is melted, 1 to 2 hours on low.
For a 5 1/2- to 7-quart slow cooker: Transfer mixture to 1 1/2‐quart souffle dish. Set dish in slow cooker and pour water into slow cooker until it reaches about one‐third up sides of dish (about 2 cups water). Cover and cook until cheese is melted, 1 to 2 hours on low. Remove dish from slow cooker.
Whisk cheese mixture until smooth, then stir in tomatoes. (Adjust consistency with hot water 2 tablespoons at a time as needed. Dip can be held on warm or low setting for up to 2 hours.).