Melt butter in skillet and saute onion, celery, parsley and mushrooms. Pour over bread cubes in a very large bowl. Add seasonings and toss to blend ingredients well.
Pour in enough chicken broth to moisten, then add giblets if using. Before adding eggs, taste to check seasonings. Add more salt and herbs if necessary. Add beaten eggs and mix together well. Pack lightly into a greased slow cooker. Cover and cook on HIGH 45 minutes; reduce to LOW and continue to cook for 4 to 8 hours longer.
You may use 14 ounces seasoned stuffing mixture in place of bread cubes, but omit herbs and salt (or add seasonings just to taste) and add about 1 extra cup of chicken broth.