My Dodge came with 16" tires and has the 3.55 gear ratio which, by factory specs, give a GTWR of 7,600 lbs.
IF it would have had the 3.92 gear ration, the GTWR would be 8,600 lbs.
If you give your Durange S/N to your Dodge dealer, he can tell you your gear ratio and GTWR.
Be wary of what the RV salesperson says. One tired to tell me a Raptor toy hauler would be no problem for my Dodge. He was right, it would be just under the weight limit ......... as long as it was EMPTY with no toys, no personal items, no propane, no water, etc.!!
2012 Fun Finder XT 276 Toy Hauler & Harley Davidson Ultra Classic with Condor wheel chock
2014 Dodge Ram 2500 AT 4X4 Crew Cab 6.4 L Hemi, 373 gears, tow package, Rambox option, Revolver X2 tonneau cover, Equal-I-zer anti-sway hitch.