Not sure about Dodges, but GM diesels at least, when in tow/haul, get several modifications: upward modified shift points and (I think) upped line pressures in trans, giving more positive shifts with less clutch pack slippage (i.e. less wear and tear and trans. heat), faster downshifts when decelerating, giving a bit more engine braking, and also changes to the lane-change blinker mode, so that when one just "bumps" the turn signal lever, it gives six "blinks" instead of three - presumably because some GM engineer thinks that when towing, we ought change lanes in a more gradual, dignified manner than when not.
There may be other, more subtle changes that I've not discovered!
The only downside that I've discovered is that in tow/haul, the Allison won't shift into top gear until at least 55 mph even at a steady speed on level ground, and if it's indeed level, I don't need the extra rpms, etc. And guess what? Some traffic engineers seem to have specified 55 mph for long stretches of American two-lane roads (not that I'd ever speed!
) so sometimes when I'm loafing along slowly on flat terrain, I will shift out of tow/haul, just to get into top gear.
But mostly I figure that it can't hurt to use tow/haul and it may help, even with a relatively light trailer in tow.