Spread the onions in the bottom of a 6- to 8-quart slow cooker. Rub the turkey thighs with the chilli powder, cumin, 1 teaspoon salt and 1/2 teaspoon pepper and place over the onions. Stir together the ketchup, sugar and mustard and pour over the turkey. Cover and cook on low for 5 to 6 hours or on high for 3 to 4 hours. The turkey should be very tender with the meat falling off the bones.
Uncover and stir. Let cool for 10 minutes. Using two forks, shred the turkey meat, discarding the bones. Stir in the vinegar and season to taste with salt and pepper. Serve on hamburger buns with pickles and/or coleslaw if desired.
Cook's Note: Choose the higher setting (and shorting cooking time) if it's more convenient for your day.