The night before cooking, layer ingredients into a slow cooker in the following order: chicken, wine, onion, bell peppers, garlic, red pepper flakes, Italian seasoning, salt, pepper, diced tomatoes, sugar, and tomato sauce. Cover and place in the refrigerator so chicken can soak up the wine, 8 hours to overnight.
The next morning, remove from the refrigerator and cook on Low, stirring occasionally, for 6 hours. Remove chicken and shred meat with 2 forks; return meat to the cooker.
Cook on High with lid slightly off to allow juices to evaporate and sauce to condense, about 2 more hours.