Stir the tomatoes, tomato paste, garlic, oregano and 1/2 tsp salt in a 6-quart slow cooker and heat on high for 30 minutes.
Mix the turkey, breadcrumbs, Parmesan, onion, egg, milk, basil, pepper and remaining 1 tsp salt just until combined.
Shape the mixture into 24 meatballs, about 1 1/2 inches each, without packing them hard.
Lower the meatballs gently into the hot sauce in one layer, spooning a little sauce over their tops without stirring.
Cover and cook on low until the meatballs reach 165°F and feel firm at the center, 3 1/2 to 4 hours.
Turn the meatballs carefully through the sauce and let them stand uncovered for 10 minutes before serving.