Preheat oven to 250 degrees F. Slice the loaf of French bread and then tear the slices of both the French and the sandwich bread into bite size pieces. Discard end pieces or save for another use. You should have about 20 cups, give or take, of torn bread. Spread the pieces of bread between two rimmed baking sheets and bake for about 20 to 30 minutes or until lightly toasted.
While the bread is toasting, melt the butter in a large skillet over medium heat. Add the onion, celery, and carrot. Turn the heat up to MEDIUM-HIGH and cook, stirring frequently for about 7 to 10 minutes. Add the garlic, parsley and mushrooms (if using) and cook and stir for another minute or two. Add the poultry seasoning, sage, thyme, marjoram, salt and pepper. Mix well to combine. Remove from the heat.
When the bread is nicely toasted, transfer it to a large roasting pan or a very large mixing bowl. Spoon the cooked vegetables over and mix well. Add the beaten eggs and stir to combine. Add just enough chicken broth to moisten the mixture, about 1 to 1-1/2 cups, reserving the rest for later. Mix well and transfer stuffing to a 6-quart (or larger) slow cooker.
Cover and set the slow cooker on LOW for 4 to 6 hours. Check on it now and then and add chicken broth throughout cooking time to achieve the desired consistency. Taste and add additional seasoning, if desired.
If you prefer a crunchy topping, transfer your removable (oven-safe) Crock-Pot dish to your oven and broil for a couple of minutes until nicely toasted and brown