In reply to the question about distance, Per year i would say we would be in that 1500KM to 2000KM max range total. We pull two decent hills on the way... were on the prairies so certainly not mountains! but there is one hill that gets me down to 3rd or 2nd gear with the 5.9. WHen i had the 15 - 6.7 with aisin it completely anihilated that hill with the same load.
Its always nice to chat with you guys/gals about this sort of info, you could call the dealership but you would get the "pamhplet answer" i consider myself experienced when it comes to towing, ive always been an overkill kinda mentality. However if an SRW would fit the bill i think that would be my next move.