Slightly off topic, but over the years I have owned and driven many of these trucks.
I had a 73 D100 with dual tanks. One in cab and one under, but only one gas cap. The filler neck had a Y in it and one branch went straight into the cab tank, and one branch went down, through the floor and into the frame tank. You simply stuck the nozzle in horizontal to fill the cab tank, or stuck it in pointing down to fill the frame tank.
On my current 75 W100 440 shortbox, it only has the in cab tank but has two gas caps. The one on the box is a painted dummy cap which is screwed in place. I had another like that too.