1986 GMC Camper Special 1 Ton Dually: Max tow capacity?

Longbed w/ Fifth Wheel hitch (unsure of make but it towed 12,000 lbs safely for several years) 454 gas 3 speed automatic (pretty sure it's a TH350) ***EDIT: Nope- it's a TH400 *** 4.10 rear diff 2...