You don't need a diesel to pull 6000 lbs. Yes, it will make for a better towing experience when you hit the mountains but the newer gas engines (6.4 Hemi, 6.0 GMs and 6.2 Ford) will be more that adequate for pulling 6000 lbs.
Diesel engine maintenance is more expensive. Expect to pay about $100 to have the oil changed ($25 filter, 3 plus gallons of oil (based on my work F550). Not sure how easy the Duramax motors are easy to work on, but the Powerstrokes are wedged in the Fords. Last year, my work truck (6.4 diesel) blew an injector pump to the tune of $4200 and last month blew a power steering hose at $600. She's an older truck with 63k miles but I'm just trying to give you an idea of the labor cost of some of these repairs when things go south.
