@ansells09 - So in all the wrangling, I'm not sure you got the answer you were looking for...
I don't know the formula for how RAM exactly determines their GVWR, but in generic terms here are some of the considerations:
-engine and engine braking, along with brakes. (higher GVWR considers the ratings for acceleration and deceleration so that's why all 3 make a difference)
-strength of frame and length of frame (cummins frames and engine mounts obviously need to be much heavier than gas engines. Also longer frame resists twisting more)
-suspension and axle ratings, as you've mentioned.
What is even more interesting to me, and I didn't realize it when I ordered my truck, is the difference between the Cummins HO, Short box 4x4 cab models -the mega cab has a higher GVWR than the Crew Cab. (12,300 for mega and 11,800 for CC)
I assume on the above example the only difference is length of frame, and that is where additional GVWR was allowed. Which is again interesting, because the length of the frame on the mega cab is in between the length of the CC shortbox and longbox, yet it gets the higher rating the same as the longbox.
Hope that provides a better answer...
B