Lightly grease the inside of your slow cooker with cooking spray or butter and set aside.
In a large skillet over medium heat, melt the butter. Add the onion, celery, mushrooms, and parsley. Cook for 5 to 8 minutes, stirring occasionally, until the vegetables are softened but not browned.
Place the bread cubes in a very large mixing bowl. Spoon the cooked vegetables over the bread, making sure to scrape in all the buttery goodness. Sprinkle with salt, sage, poultry seasoning, thyme, marjoram, and pepper.
Slowly pour in the chicken broth while gently stirring, just until the bread is evenly moistened. The mixture should be soft but not soupy. Stir in the beaten eggs until fully combined.
Transfer the stuffing mixture to the prepared slow cooker. Cover and cook on LOW for 4 to 5 hours, stirring once or twice if possible. Taste and adjust seasoning if needed, then serve warm.