Best deal is to avoid a salesperson on the lot and ask to speak directly to the fleet manager or the sales manager. You avoid paying the sales commission as these managers are on salary.
I would be buying a 2014 truck on a dealer's if I could find one and save $5,000 by doing so. Any dealer can do a search around the state and find in minutes what is on any dealer's lot.
For tires you will have more choices later with 18" wheels and an easier time finding a replacement tire in inventory if you have a blowout and need to get a tire on your travels.
Where the diesel has the advantage for towing a trailer weighing more than 10,000 lbs. is in two areas. Much faster acceleration thanks to nearly double the lb ft torque so you can merge with traffic more safely and you can pass semi's on grades with more of a cushion. The other aspect is the greater amount of engine braking from the diesel engine with its exhaust brake. I have dropped 3,000 feet in 10 miles with a heavy load in tow on a twisty mountain road and never had to touch the brakes while keeping the rig at 35 MPH. It takes a lot of the stress out of towing.
Something to check is the fuel tank capacity and whether there are options for a second tank from the factory. Titan also makes replacement fuel tanks that provide around 60 gallons of capacity without needing an auxiliary tank in the bed of the truck.
A TrailAir, Airborne or similar 5th wheel coupler with dampening will provide a much more pleasant ride for you and your passengers.
