Last year I ordered a 3500 Cummins Longhorn in SRW with cargo cam, fifth wheel prep, and rear air leveling suspension to pull our Montana 3590RD (around 15K). While a lot of folks have to have the maximum available (dually, 4:10 axles, Aisin), I ordered what I needed to pull the Montana and that included not buying the $2500 dollar Aisin which is insane. If I had plans to pull more than 17K, then a dually with Aisin would have been considered.
A SRW 3500 with 3:43 axles doesn't have any issue whatsoever tugging 15K along America's highways. Just maybe, that is why RAM's engineers made the 3:43s standard
on a SRW.
After a year of trouble free pulling, I would buy the RAM again.
