For the sauce, pour the whole tomatoes in the slow cooker. Squeeze with your hands to break them into smaller pieces. Add the diced tomatoes, garlic, parsley, oregano, red pepper flakes, and salt, to the slow cooker. Stir well.
For the meatballs, split the sausage casing down the center using a sharp knife. Turn the sausage meat out of the casing into a medium bowl. Add the ground turkey, breadcrumbs, Parmesan, egg, parsley, oregano, salt, and pepper. Use your hands to mix the ingredients just until evenly combined, being sure the sausage meat is distributed throughout (try not to overmix).
Scoop up about 1 1/2 tablespoons of meat and roll it between your hands to form little globes that are the size of a small golf ball. Add it to the slow cooker and continue with the remaining meat until you have about 20 meatballs. Nestle the meatballs down into the sauce so they are almost completely submerged.
Set the slow cooker to high and cook until firm and cooked through, 4 hours on high, 7 to 8 hours on low.
Serve in shallow bowls with Parmesan cheese to garnish and crusty bread for dipping.