Add the frozen meatballs, marinara, and diced tomatoes to a 6-quart slow cooker. Stir to combine. Cover and cook on LOW for 6 hours.
When there are about 20 minutes left in the meatball cooking time, bring a large pot of salted water to a boil. Cook the spaghetti according to the package until it's al dente. Drain the water.
Add the cooked spaghetti to the slow cooker and stir until well combined.
In a medium bowl, stir together the ricotta, Parmesan, basil, oregano, and garlic salt. Add scoops of the ricotta mixture over the spaghetti and spread it out evenly over the top. Sprinkle with the shredded mozzarella cheese.
Replace the lid on the slow cooker and cook for another 20-30 minutes until the mozzarella is melted.
Spoon portions onto dinner plates or pasta bowls, being sure everyone gets some meatballs. Garnish with chopped parsley, if desired.