Cook onion, celery, 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. Stir to coat evenly.
Pour in enough broth to moisten, and mix in eggs.
Transfer mixture to slow cooker, and cover.
Cook on high for 2-3 hours or on low for 6-8 hours.
Notes
If you open the slow cooker and it still seems a bit soupy, continue cooking with lid off for 30 minutes or so. This will help any extra liquid to cook off.