I'll try to explain it. Maximum trailer weight is really more of a marketing thing and is a phantom number. It does not take into account the options on your truck and is actually based on the lightest possible combination of options. Given your example of a QC 3. 54 Automatic in order to be able to tow the maximum trailer weight of 10,500 lbs it would be a short bed truck, 2wd, with the lightest interior (cloth or even vinyl if it's available) the smallest engine available (the 5. 9 gasser I believe) completely stripped (no power windows, door locks, clearance lights, bedliner, fog lamps, carpet, etc, etc), only a driver sitting in it (they probably estimate you around 170 lbs), no passenger, no luggage, no bags, not even a briefcase, no fuel in the tank (!) - I think you get the picture.
Now for the real world, to be legal, what you have to do is get the actual weight of your truck along with your load in the truck - you, the wife, kids, dog, clothes, full tank of fuel, the trailer hooked up (the pin weight on a 5er, or the tongue weight on a TT) - the weight on both of the truck axles has to be less than the GVWR of 8,800 lbs. The weight on each axle individually has to be less than the GAWR ratings listed in your door jamb. On the trailer, with all of your stuff loaded into it and the tanks loaded how you plan on towing it, the weight on both of the trailer axles has to be less than the GAWR rating on the trailer ID tag (usually on the drivers side front) and less than the GVWR rating on that same tag, Then all of the axles put together have to be less than the GCVWR that your truck is rated for.
In the end the answer to your question is that there is almost no way that you will EVER be able to tow a trailer that weighs the max trailer weight that's listed. As an example my truck with me, my wife, daughter, the normal stuff carried in it, and dog with a full fuel tank weighs 7,800 lbs (2500 QC LB 5sp 4x4 with 4. 10's). It's rated at 8,800 GVWR and 18,000 GCVWR with the max trailer weight listed at 11,900. So the TRUE max trailer weight would be 10,200 lbs (with the trailer fully loaded, not empty) and the tongue weight or pin weight couldn't be over 1,000 lbs. Which means based on the recommended tongue weight of 10% on a TT I could only tow a 10,000 lb TT which would put me 200 lbs below the max GCVWR. Based on the recommended pin weight of 15% on a 5er I could only tow a 6675 lb fiver. Which would put me 2525 lbs under the GCVWR but I would be at the max 8,800 GVWR!!
Now that you are probably completely confused (I hope not) what it comes down to is forget the max trailer rating completely - just ignore it. If you want to be legal make sure that fully loaded you are under the GVWR of the truck and trailer, under all of the GAWR's (axle ratings) of all 4 axles, under the tongue weight rating in the case of a TT, and under the GCVWR of your truck (with the trailer attached). Your limit is the LOWEST of any of those numbers.
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