Spray a slow cooker with nonstick cooking spray. Add the water and liquid smoke (if using) to the bottom of the slow cooker.
Prepare the ribs by removing the shiny, thin membrane that covers most of the backside of the ribs. To do so, use a knife or fingernail to pry up a corner of the thin membrane from one edge of the ribs. Grab that with a paper towel (for grip) and slowly pull it up. Once a good portion of the membrane is free, you should be able to pull the rest off in one quick motion.
Pat the ribs dry.
In a small bowl, stir together the brown sugar, chili powder, salt, cumin, paprika, garlic powder, and onion powder.
Rub the seasoning mixture onto both sides of the ribs. Place the ribs in the slow cooker. If needed, cut each rack into halves or thirds and stack them so that they fit in your slow cooker. Pour 1 cup of barbecue sauce over the ribs.
Cover and cook on LOW for 8-9 hours (best texture), or on HIGH for about 4 hours. Ribs are done when the meat reaches an internal temp of about 195-205°F.
Preheat the broiler. Transfer the cooked ribs to a foil-lined baking sheet. Brush generously with more barbecue sauce and broil until caramelized and starting to char, about 2-4 minutes. Cut into individual ribs and serve with extra bbq sauce, if desired. Garnish with chopped fresh parsley just before serving.