Place the ground beef and Italian sausage in a large bowl. Add the breadcrumbs, egg, chopped roasted red peppers, Parmesan, Italian seasoning, salt, and black pepper.
Mix with clean hands or a spoon until the ingredients are evenly combined but not overworked.
Shape the mixture into a loaf that will fit inside your slow cooker. Lightly grease the slow cooker insert if desired, then set the loaf in the center.
Spread the marinara sauce evenly over the top of the loaf.
Cover and cook on low for 6 hours, until the internal temperature reaches 160°F.
Let the meatloaf rest for 10 minutes before slicing and serving.