Melt butter in a large skillet over medium heat.
Add the onion, celery, mushrooms, and parsley. Sauté until the veggies are softened, about 5 to 8 minutes.
Place dry bread cubes in a large bowl. Add the cooked vegetable mixture, salt, sage, poultry seasoning, thyme, marjoram, and pepper. Mix well.
Pour chicken broth over the bread mixture a little at a time, stirring to coat. The mixture should feel moist but not soggy.
Fold in beaten eggs until evenly combined.
Transfer the mixture to the slow cooker. Cover and cook on High for 45 minutes.
Reduce the heat to Low and let cook for 4 to 8 hours. It's ready when the flavors are fully blended, and the texture is soft yet firm.