Place bread in a large bowl. Pour the milk over the bread and let stand 10 minutes.
Coarsely chop 1 of the carrots and cut the remaining 2 carrots into chunks. COarsely chop 1 onion and slice remaining onions. Set aside carrot chunks and sliced onions.
Place chopped carrot, chopped onion and celery in a food processor; pulse into finely chopped. Transfer to a bowl with bread mixture. Add the turkey, egg and salt; mix with hands until combined. Shape into an oval loaf and place in a slow cooker.
Arrange the potatoes, mushrooms and reserved carrot chunks and onion slices around the meatloaf. Cover and cook until the meat and vegtables are fork-tender, 4-5 hours on high or 8-10 hours on low.
Transfer the meatloaf to a platter and cut into 6 slices. Spoon vegetables around the meatloaf.