Cut sausages apart into individual links if necessary. Cook links in a large skillet over medium heat until browned on all sides. Set sausages aside and reserve the pan drippings.
Pour whole tomatoes, crushed tomatoes, and tomato sauce into a large saucepan with a lid. Stir in Parmesan, garlic powder, Italian seasoning, sugar, and onion powder. Bring to a boil, then add sausages, pan drippings, green peppers, and onions. Reduce the heat to low, cover, and simmer until vegetables are very tender and sauce has thickened, about 6 hours. Serve on Italian hoagie buns.