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

That's a sweet truck. I love that body style. I would have to bet the transmission is a TH400, not a TH350.
If it towed 12,000 for years safely I wouldn't be afraid to keep doing it.
If you're talking about a FW i would think you would want no more than 3,000 pounds on the pin. That's about a 15,000 pound trailer. However, I would suspect that truck to be very slow and drink gas like mad with that kind of weight.