GMC YUKON with Auxiliary Gas Tank

Recently bought a used 2001 GMC YUKON XL with an auxiliary 24 gallon gas tank. When the gas gauge shows almost empty we can only put about 29 gallons in it. Does anyone know how the two tanks work? Does the gauge only register one of the tanks (which one)? You fill at the rear tank, is it a gravity feed to the front or is there a pump? We’re afraid to keep driving with the gauge on E since we don’t know how it’s supposed to work. The GM dealer we took it to couldn’t find any info on how it’s supposed to work. There is no switch to switch tanks so they are connected in some way.

I have one with a normal tank. It takes about 29 gallons when empty. Are you sure there are two tanks?

Get under the GMC and see if you can physically see the separate tanks. Let us know what you find.