I'm with you on dealer using as a toy...seen more than one kid of the owner or sales manager dog a vehicle or two.Something is wrong with it. The diesel should easily get 20+ on the highway when driven kindly. Are you positive the truck is unmodified? Is sounds like it was the dealer's play toy before you bought it, so it's possible whoever was driving it modified it and then did an RTS before sale and messed something up.The Titan and Suburban are driven in like manners (maximum speed solo 70 MPH -- maximum with trailer 60 MPH), so it is hard for me to understand why the Nissan's fuel economy is so low -- needless to say with the premium now charged for diesel fuel, the cost of operating the Nissan is significantly more than the Suburban. Hopefully the Nissan may improve with more miles on the odometer, but my optimism is fading.
Do you have access to an OBDII scan tool? OBD Fusion with the $10 nissan plugin can read the codes from all of the on-board computers on the Titan. It might tell you something. I would suspect something with the emissions system before anything else. EGRs and Turbos are known to be prone to infant mortality on these engines, too.
Some parameters you definitely want to check are the DPF load and regen state. A Titan will get 10-12mpg in regeneration regardless of driving manner.