Break the cooked carcass into pieces that fit in an 8-quart slow cooker. Add stock onion, stock carrots, stock celery, bay leaf, peppercorns and the 12 cups water, keeping below the cooker maximum-fill line; cover.
Cook on high for 1 hour, then on low for 6 to 7 hours. Keep the reserved cooked turkey refrigerated while the stock cooks.
Lift out large bones with tongs and strain stock through a fine-mesh sieve into a large heatproof container. Discard all bones, peppercorns and spent vegetables and skim excess fat.
Measure 8 cups strained stock into a 6-quart soup pot, topping up with the reserved water only if necessary; cool and refrigerate or freeze any extra stock promptly. Add lentils, fresh carrot and salt and bring to a simmer.
Simmer for 15 minutes, add turnip and continue for 20 to 25 minutes until lentils and turnip are tender.
Stir in the refrigerated roast turkey and simmer for 5 to 8 minutes until it reaches 165°F. Add lemon juice and serve, checking that no bone fragments entered the strained stock.