ahfox wrote:
The mechanic at the Chevy garage says everything checks out OK electronically...
What the h3ll does "electronically" have to do with the fuel filler hose? The mechanic sounds like he's in the wrong field.
In addition to what others have suggested, also check the vent line. On my old class C, the vent hose runs parallel to the fill hose; one of the mounts had broken and the line dipped down lower than the fill hose. That allowed fuel to pool in the low spot of the line, effectively blocking the vent line. Resulted in the exact scenario you are describing.
On the other hand, my wife's '06 Tahoe is a bear to fill regardless, and the pump clicks off constantly. On that, it is simply a horrible design, with an actual filler hose that is only 1" in diameter and can't keep up with the flow. Some pumps are better than others, but rarely is it not a problem.