Combine vinegars, salt, pepper, cloves, bay leaves and mustard seeds in a large zip lock bag. Add roast and refrigerate about 3 days, turning each day.
Place roast and marinade in slow cooker and cook on low for about 8 hours. Remove roast to a platter and keep warm. Separate fat from liquid. Stir in crushed gingersnaps and bring to simmer until well blended and slightly thickened. Slice roast and serve with gravy. Serves 10.