Place ingredients in slow cooker: Place chicken in an even layer in a 5- or 6-quart slow cooker. Top with onion, gravy mix, dressing mix, salt, banana peppers and pickling liquid, and butter. Cover and cook until chicken is very tender, about 2 hours on HIGH or 4 hours on LOW.
Shred chicken and serve: Remove chicken from slow cooker with tongs or a slotted spoon, and place on a cutting board; coarsely shred chicken using 2 forks. Return chicken to liquid in slow cooker. Serve over mashed potatoes or rice, and garnish with parsley.
