When I was in the car business, I NEVER heard of any tires being purposefully deflated to smooth out the ride. When people were buying a 3/4 or 1 ton truck, they knew that they were going to get the ride of a vehicle that was designed to have a lot of use. When you have a truck that is capable of towing in excess of 15k pounds and well over 2k pounds in the bed, then naturally the suspension is going to be firmer that a SUV or a car.
I'm beginning to think that you have buyers remorse because while changing the tire pressure will possibly help out a little, it's not going to make a huge significant difference. But since you haven't even tried to do that, or at least haven't responded that you did and if it made any difference, then I don't know that there's really anything that's gonna please you.
You said in a previous post that your back doesn't hurt on the short drives, just the long 3+ hours in the seat. That's not the salesman's fault. He probably took you on a 15-20 min test drive, but that won't replicate a long road trip by any stretch of the imagination.
I think that you're trying to cast all the blame on the salesman/dealership because you bought more truck than you need and the ride is stiffer that you had expected once you got some time in the seat.
You have also sated the you have owned a Dodge 2500 before, so you should have been well acquainted with the ride characteristics of a truck like the 3500. true, it might be a tad bit stiffer because it is 4x4 and brand new, but from everything I have read, the ride is smoother now as compared to the 2008 model you had.
Hell, I didn't realize how much road noise my tires are capable of producing based on the different road surfaces, but that's not the salesman's fault and I wasn't duped into buying the Escape. However, they are the original Michelin tires and I have over 65 K miles on them, and I can probably get at least another 30K out of them before I need to worry about replacing them, I am willing to make the sacrifice of loud road noise on some surfaces.
Even my wife knows that the ride in big truck will be significantly rougher than in a car. You were not duped, mislead or outright lied to. You have worked yourself into a tizzy when all you have to do is take more breaks while on the road and get out and stretch.
FYI, I have worked for a back surgeon for the last 4.5 years
