Add the meatballs: Place the frozen meatballs in the bottom of your slow cooker. No need to thaw them first.
Add the sauces: Pour the entire jar of marinara sauce and Alfredo sauce over the meatballs. Stir gently so the meatballs are coated.
Slow cook: Cover and cook on LOW for 4–5 hours or HIGH for 2–3 hours, until the meatballs are heated through and tender.
Cook the pasta separately: About 20 minutes before serving, cook the rotini according to the package directions until just al dente. Drain well.
Combine: Add the cooked pasta to the slow cooker and gently stir it into the meatballs and creamy tomato sauce. If the sauce is too thick, add a splash of water or chicken broth.
Make it cheesy: Sprinkle the mozzarella and Parmesan over the pasta. Cover and cook for another 10–15 minutes, or until the cheese is completely melted.
Serve: Stir everything together and top with Italian seasoning or fresh parsley. Serve hot with extra Parmesan if desired.