In a medium saucepan cook sausage and onion over medium heat until sausage is brown; drain fat. Transfer sausage mixture to a 3-1/2- to 4-quart electric slow crockery cooker. Stir in cheese, tomatoes, and wieners.
Cover and cook on high-heat setting for 1 to 2 hours or until cheese is melted, stirring after 1 hour. To serve, keep dip warm on low-heat setting, stirring occasionally. Serve with tortilla chips. Makes 18 servings.