You can forgo the rear camera as it will be of no benefit with the 5th wheel and GM charges $400 for a wiring cable to hook up a third party camera (Ford charges less than $50).
A Trailair or similar shock absorbing king box will make a big difference in the comfort of anyone in the cab by isolating the trailer's motion.
I would check whether Titan has a replacement tank for your Duramax. I found out too late that they did not make one for my 2011 Duramax truck and the extra 24 gallons would be great to have on trips when the truck is getting around 10 MPG.
The GM tow mirrors are important and I got the dealer to include them on my truck at no charge. Also got him to throw in a GM bed liner and GM chrome side steps (very important as sill height is 24 inches above the ground), and a GM aluminum diamond plate tool box.
Be sure to check that the dealer's people have topped off the DEF tank and the ATF as this is a common oversight.
Although the 2011 and later trucks have a fuel warmer to help with B20 biodiesel you are better off not using it at all and if you do use it do not switch back and forth between B20 and standard diesel as that will cause the worst problems and they are not covered by the warranty.
Allison transmission's outer spin off filter should be changed at 750 miles and then at the regular change interval.
Front end has 9 grease points and they should be greased every 3,000 miles. I add the grease myself as the crankcase oil is changed at 7500 mile intervals. Each time I add grease quite a lot goes into the fittings. A pistol type grease gun with a 90 degree fitting makes the job a lot easier.
