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.