Does the door sticker list the tire size and load rating it's giving the tire pressure for? I ask because you said it was a custom truck, therefore a different size of tire or a different rating of tire may have been put on afterwards.
What is the tire manufacture's recommended tire pressure on the side of the tires?
I would expect a 3500 dually to run more than 65 PSI in the rear tires. If you do run them with only 65 psi in them, for the first trip I'd be checking them often for over heating.
Your post should also list the tire size and load rating to help people.
I'm not sure why you have to insist the dealer fill them to any certain pressure? Stop by the nearest service station and fill them to what ever you want or use your compressor at home. You can be on your way and gone before you get done arguing with the dealer.
Bill