There would be no dealer in my area that would have a 2003 truck on their lot, unless they were a small used car dealership. The regular new car dealers would take a trade-in like that directly to auction. Remember, no matter how many miles that truck has on it, the 2015 models are out so it is 12 years old.
Since the difference in values of what you are purchasing and what you are trying to trade is so great, I would sell your truck and then start working cash deals for your new vehicle.
One of the few advantages of trading, at least in my state, is that you only pay sales tax on the difference.