In a large skillet over medium-high heat, crumble and brown the Italian sausage until fully cooked, about 6-8 minutes. Drain any excess fat and transfer the sausage into the slow cooker.

In the slow cooker, combine the cooked sausage with the spaghetti sauce and Italian diced tomatoes. Stir well to ensure the sausage is evenly coated.

Cover the slow cooker and set it to low heat for 6-8 hours.

Carefully stir in the frozen or refrigerated tortellini and cover the slow cooker again, cooking for another 15 minutes on low heat.

After 15 minutes, sprinkle mozzarella and cheddar cheeses generously on top and let it cook for an additional 15-25 minutes.

Once the cheese is melted and bubbly, carefully spoon the Cheesy Crockpot Tortellini onto plates.