Preheat oven to 325 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 the end pieces or save them 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 15 to 20 minutes, or until lightly toasted, tossing with a spatula about halfway through.
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, until tender. 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 the bread and mix well. Add the beaten eggs and stir to combine. Add just enough chicken broth to moisten the mixture, about 1 to 1½ cups, reserving the rest for later. Mix well.
Coat a 6-quart (or larger) slow cooker with nonstick cooking spray and add the stuffing. Cover and set the slow cooker on LOW for 4 to 6 hours. Check on it now and then and add broth throughout cooking time to achieve the desired consistency. Taste and add additional seasoning, if desired. Set the slow cooker to the KEEP WARM setting, if it has one. Alternately, unplug the slow cooker and keep it covered until ready to serve.
If you prefer a crunchy topping, transfer your removable (oven-safe) Crock Pot insert to your oven and broil for a couple of minutes until nicely toasted and golden brown.