Spray the inside of a slow cooker with nonstick cooking spray. Place the chicken in the slow cooker and sprinkle with the Italian dressing mix.
In a large bowl, combine the marinara sauce, condensed soup, and diced tomatoes. Pour the sauce over the chicken.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours, until the chicken reaches an internal temperature of 165°F. Remove the chicken to a cutting board and cover loosely with foil to keep warm. While the chicken rests on the cutting board, add the cream cheese to the slow cooker, stir, and cover. Continue cooking the sauce for about 10 more minutes, whisking regularly, until the cream cheese melts and smoothly incorporates into the sauce.
Meanwhile, shred the chicken with two forks. Return the chicken to the creamy sauce and stir.
Toss the chicken and sauce with cooked spaghetti or spoon over rice. Garnish with grated Parmesan, crushed red pepper flakes, or fresh herbs, if desired.