Hello @ColoRetiree and welcome to GM Authority!
I also live in Colorado and have drove the 2014 Silverado when it came out as a company vehicle. Though I didn’t do much towing with it, I did use it to go on many a ski trip in the mountains through I-70. The truck had the 5.3L and 6-speed transmission, but the 6.2L behaves similarly, albeit with more power.
Now, more pertinent to your question: the way I have always understood it, AFM doesn’t actually stop the valvetrain of the deactivated cylinders, but rather stops “feeding” them, thereby attaining the fuel savings. There is a lot of misleading literature out there saying that the inactive four cylinders are “shut off”, but I believe that’s an over-generalization. So, I don’t think AFM will impact engine braking for you. Does that help at all?
