Stir tomatoes, tomato paste, garlic, oregano, and 1/2 tsp salt in a 6-quart slow cooker. Heat on high for 30 minutes.
Mix turkey, breadcrumbs, Parmesan, onion, egg, milk, basil, pepper, and remaining 1 tsp salt just until combined.
Gently shape into 24 meatballs, about 1 1/2 inches each; do not pack tightly.
Gently lower into hot sauce in one layer. Spoon sauce over tops without stirring.
Cover; cook on low for 3 1/2 to 4 hours until meatballs reach 165°F and centers feel firm.
Carefully turn meatballs through sauce. Rest uncovered for 10 minutes before serving.