Break carcass to fit 8-quart slow cooker. Add stock onion, carrots, celery, bay, peppercorns, and 12 cups water, staying below maximum-fill line. Cover.
Cook on high 1 hour, then low 6 to 7 hours. Keep reserved turkey refrigerated meanwhile.
Tong-lift large bones; fine-mesh strain into large heatproof container. Discard bones, peppercorns, and spent vegetables; skim excess fat.
Measure 8 cups stock into 6-quart pot, adding reserved water only if needed. Promptly cool and refrigerate or freeze excess stock. Add lentils, fresh carrot, and salt; simmer.
Simmer 15 minutes; add turnip and cook 20 to 25 minutes more until both tender.
Add refrigerated turkey; simmer 5 to 8 minutes to 165°F. Add lemon juice; serve, checking no bone fragments entered stock.