Combine ground round and turkey in a bowl; mix until fully combined. Stir in bread crumbs, soup mix, parsley flakes, black pepper, and garlic powder. Add eggs, 1/4 cup ketchup, and mustard; mix well. Form meat mixture into a loaf. Place in the slow cooker. Cook on High until no longer pink in the center, about 6 hours. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).
Mix 3 tablespoons ketchup and brown sugar together in a small bowl to make the glaze. Spread over the meatloaf; continue cooking until glaze is set, about 1 hour more.