Sounds like your dealer knew what hitch to sell you. You will thank him.
Af far as placement that is a "hit" and miss deal. Take dimensions from the outer edge of the front cap to the center of the kingpin and then from the back of the cab to determine where the hole in the 5th wheel should go. Hook up and see where you are. The farther forward you can mount your hitch the better.
2015 RAM LongHorn 3500 Dually CrewCab 4X4 CUMMINS/AISIN RearAir 385HP/865TQ 4:10's
37,800# GCVWR "Towing Beast"
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2016 MobileSuites 39TKSB3 highly "Elited" In the stable
2007.5 Mobile Suites 36 SB3 29,000# Combined SOLD