Melt butter or margarine in a skillet over medium heat. Cook onion, celery, mushroom, and parsley in butter, stirring frequently.
Spoon cooked vegetables over bread cubes in a very large mixing bowl. Season with poultry seasoning, sage, thyme, marjoram, and salt and pepper. Pour in enough broth to moisten, and mix in eggs. Transfer mixture to slow cooker, and cover.
Cook on High for 45 minutes, then reduce heat to Low, and cook for 4 to 8 hours.
Note: To make the slow cooker stuffing in the oven, prepare as directed using the full amount of broth. Transfer to a 9x13 inch baking dish or other large casserole dish. Bake uncovered for 45 minutes to 1 hour at 350 degrees F (175 degrees C). This recipe is designed for use in a standard 4 quart slow cooker. Larger slow cookers will also work.
Easy Cleanup: Try using a liner in your slow cooker for easier cleanup.