I’m beginning to wonder if GM full size SUVs and trucks only have evap OBD codes available in the ECM...
Finally after a little over a year since my wife bought her current one, it threw P0455 code today.

Her 07 had nothing but them (and probably ALL of them, too... uggghhh) when ever it threw a code or two, my Silverado throws occasional P0454 and I don’t remember a time any of previous ones would’ve thrown any other than an evap code.
Cleared it and hope it won’t come back anytime soon. And of course it threw it just when I was planning to get it inspected in the next couple of days.