Place your roast in your slow cooker. Pour the Campbell's sauce over the top. Cover with the lid and cook on low for 8 hours.
Transfer your meat to a serving dish.
Strain the remaining liquid into a medium sized pot. Whisk in 1 cup of water.
In a small bowl stir 1 tablespoon water with 1 tablespoon cornstarch until dissolved. Stir into your gravy. Heat to a boil, stirring consistently, and cook until slightly thickened. Serve over potatoes and meat.