Prepare the slow cooker by placing the rotisserie chicken carcass, with skin on, into the pot.
Add the chopped carrots, celery, smashed garlic, bay leaf, black peppercorns, salt and a few sprigs of parsley to the slow cooker.
Pour in the water and stir in the apple cider vinegar.
Cover and cook on low for 12 hours, allowing the flavors to develop.
Remove the large solids using tongs or a slotted spoon, then strain the broth through a fine-mesh sieve into a large bowl.
Transfer to storage jars and allow to cook 30 minutes before storing.