Fuel tank size

[slotard]

I have a '73 C20, 350/350, Camper Special.

From the factory it appears it had dual tanks, both on the passenger side. At one point the rear tank was removed, so it now has one running from a foot and change in front of the wheel well to midway up the passenger door.

How big is the tank? I thought I was down around 10gal and just put in a full 20 before stopping it myself. I'd like to know if the pump is screwed up or if I just have a bigger tank than I thought - I thought it was 18.