You're confusing some terms here, and perhaps that is why you are getting suggestions all over the map.
The payload you asked about is how much weight the vehicle can CARRY on it's four wheels. It is a subtraction of how much the vehicle weighs empty, taken from the manufacturer rated GVWR (Gross Vehicle Weight rating). For example, my truck weighs 7000#, and has a 10,000# GVWR set by RAM and therefore has a payload capacity of 3000#. It also has Gross axle weight ratings of 5500# front and 6500# rear. Those are denoted by "GAWR". Notice we haven't even talked about trailer towing yet.
In regards to trailer towing, there are two numbers to consider. One, most people are familiar with, is the max towing capacity. This is the number they flaunt on the commercials. But there is another number which is very closely related called GCWR, or Gross COMBINED Weight rating. This is the maximum the tow vehicle and trailer can weigh when added together. I'll use my trauck again as an example.
Max trailer rating Ram 2500 with 4.10 gears - 15,470#
Max GCWR - 22,500#.
If my truck had 3.73 gears, both those numbers (max towing and GCWR) drop 3000# each to 12,470# and 19,500# respectively.
So - I think the OP is talking about GCWR or max tow, in which case gearing is very important. The manufacturers (until SAE J2807 comes into effect - google it) all had their own internal tests for how they set a max tow rating. As an example, manufacturer "A" will tell their engineers that for a given tow rating, the vehicle must be able to achieve say, 45 mph on a 7% grade and the coolant temp must not exceed 240*F. So, they tow different weights until they can satisfy tests like those. Then the advertising dept gets into the fray and looks at what manufacturer "B" says their truck (in the same class) can tow...and....then the fun begins.
