I don't have a definitive answer for you, but I can tell you that the previous generation Yukon XL 2500 came with (2) tanks. The main is approximately 30 gallons and the reserve is about 7 gallons. Total capacity is rated at 37.5 gallons IIRC.
Similar comment, your fuel transfer pump that moves the secondary tank contents to the main tank could be failing. I am not aware of more than one sensor, which is in the main. I don't believe there is any way to determine the actual level of the secondary tank. When the primary gets low the secondary is continuously transferred to the primary.
The transfer pump only transfers from the secondary to the primary, never the other way around. The secondary will fill when you add at the pump and I believe it will fill last. i.e. when the level in the primary gets near full it will spill over into the secondary. I'm not positive about this but is consistent with my experience having owned this truck for nearly 15 years.
Another issue may be a partially clogged tank vent. When you stop pumping do you wait a little and try to pump again or do you assume the tank is actually full? As you pump the backpressure in the tank increases as the vent path is ultimately back out the filler neck (constricted). Backpressure is how the automatic fill stop works; when it hits a limit the handle clicks off. If the vent is partially constricted it won't be able to vent off quickly enough and you may think the tank is full when it's really almost full. Where it clicks off would be a function of the pump rate, the vent rate, and the tank level. If you like second-order differential equations we can have a more detailed discussion but I assume you don't...
I know we're told not to "top of your tank" but you may want to see just how much you can squeeze in there by allowing a few minutes to vent before you try to put more fuel in your tank. You may have to repeat this process more than once. What sometimes happens is the fuel vent filter medium will come loose and lodge in the vent line hose from the tank back to the filler neck. The filter material is a medium of small carbon balls. Let us know if you suspect this is the case as I can provide additional information if you care to know. Ironically, topping off your tank can be a contributing factor to filter media failure.
The low-level sensor works independently of the level sensor and the fuel gauge and is indicative of a true low fuel condition regardless of what may be indicated at the dash. If your low level-light comes on you should have about 5 gallons total remaining in the primary. Time to fill up as you likely have less than 50 miles until empty. If your level sensor or gauge ever fails, you can essentially drive off the low-level light.
Don't drive the truck to empty as the fuel pump is cooled by the fuel in which it sits. This is a good way to overheat a pump and reduce its useable life or cause outright failure.
