I have the info on my '75 GMC, and you could get either the Camper Special or the Trailering Special option in 1975.
Camper Special (Option Z89) ,- Dual alternator connection to a relay that disconnected the aux circuit (Red wire) when the truck was shut off. The Aux circuit was designed for accessories that needed 12 volts all the time, even if the truck was turned off. Taillight/brake wires also ran from the engine compartment back to a point under the truck and up to a connector for the camper at the front of the box. Dual batteries (Option TP2) were also an option, but you could have camper special and AUX circuit even if there was no dual battery.
Trailering Special - wiring that ran to a trailer connector at the rear bumper, including an AUX wire connection. Other than the blue color for the AUX wire, I can't tell where it connected. It looks like it was just pre-wired with a trailer brake wire.
Most of these trucks also had heavy duty cooling, but it was a separate option. Due to the GVW (8200), mine also has heavy duty 13" rear brake drums and a full-floating rear axle. I also have to buy E load rating tires in current sizes to get enough capacity for my 5600 lbs GVW rear axle.