不需要N，我能证明我对了 。。。 lol

here is allison 1000/2000 series operator's manual:

http://www.rvtechlibrary.com/transmissions/1000_2000_operators.pdf

on page 14-24, there are clear instructions on how to operate those trannies, including 2500 model.  there is nowhere ever mentioning that from D to R must stop at N.  The instruction says:  

Completely stop the vehicle and let the engine return to idle before shifting from a forward range to R or from R to a forward range.

what is idle?  here is the definition https://en.wikipedia.org/wiki/Idle_(engine).  notice, idle is not the same as neutral (N):

Idling refers to running a vehicle's engine when the vehicle is not in motion. This commonly occurs when drivers are stopped at a red light, waiting while parked outside a business or residence, or otherwise stationary with the engine running.

so, from d going n before going r will not harm anything, just not required.