On our '74, the large gold "Control Panel" is in the locker directly over the burners. On the right hand side of the Control Panel is a squarish knob the slides up and down almost the entire height of the Control Panel.
It opens the vent cover hatch for the range hood on the outside of the trailer, and and the end of its travel, it switches on the fan. It's purely mechanical, pulling on a solid steel wire which is encased inside a coiled steel wire sheath.
Mine was binding to the point where the inside cable had bent and no longer either opened the outer vent hatch or triggered the electrical switch. I took it apart, squirted enough liquid lubricant (I think it was spray graphite grease) into the sheath, worked the cable a number of times, and it eventually operated smoothly.
The fan is noisier than a banshee howling though, so I think I will have a closer look at it this summer to see if there is any way to lube it. Those fans are still available, but at silly prices.
If it's to be, it's up to me.