If you have an RS with AWD I think it comes standard with the tow package. Does your vehicle have the built in receiver and 7-pin wiring? I think the thing about the alternator I saw on the "build your own" part of the Chevy website when I was playing around with it. It's in the info for adding the tow package. If you download the Blazer brochure there is also info about towing/trailering specs where it compares models.
Also, if you use the compare trims tool on the website and compare RS AWD and RS FWD you can see the FWD comes with a 155amp alternator and the AWD comes with a 170amp. It also confirms what I saw in the brochure that AWD comes with heavy duty cooling, hitch guidance, and hitch view.