I have a 2005 Dodge 2500 diesel quad cab, standard as the day it left the factory.
great truck, runs good, drives good, gets good mpg, but you cannot lock out OD in this model, do not know if you can lock out OD in a 2003 model.
However, I would not want to hook a 12,000 trailer behind my truck.
you get a 3.73 differential standard, for that kind of weight you need 4.10 or even a lower ratio.
my truck has only 310 hp and 625 # of torque,
I do not think the truck you are looking at has that much, as they kept increasing the hp and torque ever few years to keep up with chevy and ford.
who knows what the new dodge trucks have today, but its far more than my old truck.
I do not think you will be satisfied with that truck and trailer combination.
I pull a 6500# bumper hitch trailer, I know its back there, but the truck pulls it well and I can increase speed going up a hill, so its not overloaded.
for a trailer like the one you mentioned, you may need a 5500 series truck, to safely pull it and stop it.
good luck,
do not "under truck" or "over trailer" you will be an unhappy camper!!!!!!!!!!!