Welcome aboard Don. That's a lovely Dually ya got there.
Just about everything you could hope to know about your truck's factory options can be found here at the GM Heritage Center. Scroll down to page 141 for fuel tank specs... It appears yours would come stock with 20 gallon tanks.
https://www.gmheritagecenter.com/doc...olet-Truck.pdf
Edit: For reference, your truck is referred to as a CC30903 in the info sheets. On page 141 it lists 1 20 gallon tank as standard, and an additional 20 gallon tank as optional for your rig...