Over medium high heat, brown ground beef and onion. Crumble beef as it browns. Drain if necessary.
Add ground beef and onion, marinara sauce, diced tomatoes, garlic, oregano and basil to slow cooker. Cook on High heat for 3 hours or on low heat for 6 hours.
Mix all the different kinds of cheese together.
Stir in tortellini and half of the cheeses. Sprinkle remaining cheese over the top.
Cover and cook for 15-20 more minutes or until tortellini is tender.
Serve while hot!