I had the same experience with the various brochures and towing guides from the manufacturers being unreliable for cargo and tow ratings. If you're lucky they will be within 400#. The doorjamb sticker (or sometimes in the glove box) is the most accurate data and will usually be specific to the VIN or configuration and trim level.
The loaded up trucks suffer in cargo capacity, no doubt about it. You're trading off cargo capacity for options, crew cab, etc (and you carry it around all the time). And at least with GMC, going with the heavier engine options also deducts from cargo capacity. In a GMC, if you care about cargo or towing capacity, best option is a lower trim level with the smaller V8 and avoid crew cab. That can gain 200-500# in extra capacity for cargo and towing.
I hear a lot of people proclaim that the bigger engines are better for towing. Well, they might feel stronger when towing, but they also knock down the towing capacity. I think there is a couple hundred pound loss in tow rating with the larger V8 in GMC.
