Rinse rump roast and pat dry. Sprinkle the onion soup mix in the bottom of the slow cooker. Place the rump roast in and spoon cranberry sauce around and over roast. Cover and cook on low 10 to 12 hours. Remove roast and allow to rest while you thicken with gravy. Turn the slow cooker up to high. Blend the butter and flour into a paste and whisk it into the gravy. Cover and cook on high for about 10 minutes, until thick. Slice roast into 1/4-inch slices and serve with gravy.