You need to do more math on the "Gas is cheaper than diesel" theory. I run a F-350 hotshot truck powered by a V-10 that averages 6.5 mpg. Most guys running the diesels (Fords and Dodges) are getting 11 mpg. Using current fuel prices posted at the top of the forum page, the cost of gas is 56...
Think about how most class 6 and 7 trucks are used. Delivering furniture or lumber by a $8/hr idiot who doesn't give a flip about the equipment. Daily abuse like that will shorten the life of any motor vehicle.
All the other posts are dead on about maintainence. That's what adds life to a truck.
I've been hotshotting for a year and have never been asked where I slept last night. Just don't EVER log on line 2. It will be the death of you. ALWAYS log line 1 when off duty if you don't have a DOT legal sleeper.
Google up MBE 906 and somewhere in the list should be an article about the life of these engines. I read some paper a year ago where Detroit engineers took a MBE 906 that had over 670,xxx on the clock and tore it down to the block. They measured everything against 'new' part specs and everything...