Look, I get it: There's a rule that if you live west of Kansas you can't go camping without 1500 pounds of stuff in the trailer and another 1500 pounds of people and more stuff in the tow vehicle, and for all that stuff you really, really need the biggest truck that you can get your wife to let you buy. And, it needs to be built on a proper steel channel truck chassis so that you could drill bolt holes in it anywhere you want to, not one of those sissy unibodies.
But that doesn't mean we all need big pickup trucks, just because it's what guys do out West in the mountains or the desert. I promise: I won't have a 600# tongue or a 4500# TrailManor on the road, and if I ever go off road with the TrailManor, it won't be intentional.
Here's what Toyota is saying about towing these days. This is different from a few years ago, and it provides a lot more detail (the term Gross Combined Weight Rating only occurs in the glossary).
One inference from how hard it is to get to the GCWR is that maybe Toyota treats axle weights, gross vehicle weights, and trailer weights as the important parameters and not the GCWR. We've seen them bend GCWR smoothly around that extra few hundred pounds for 4 wheel drive. GCWR is not marked anywhere on my vehicle, only buried deep in the owner's manual which, now in the scanner instead of the glove box, is no longer in the vehicle. The axle weights, on the other hand are stamped on the door post and are things anyone can measure at a truck scale without having to do any arithmetic.
http://www.toyota.com/pdfs/towguide_Part1.pdf
http://www.toyota.com/pdfs/towguide_Part2.pdf
http://www.toyota.com/pdfs/towguide_Part3.pdf
Here's the best advice to be giving people about tow vehicles, as quoted by Harvey from the Toyota website:
"Do not exceed any Weight Ratings and follow all instructions in your Owner's Manual."
Toyota's towing guide
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