Dodge with the Cummins turbo diesel...
Not because it`s "Dodge" but the Turbo diesel is a "commercial" diesel engine with similar construction to big trucks. Ford and Chevy diesls are thirsty and expensive to fix with their complicated computer controls and comparatively fragile construction..
Pull a 3 axle 5th-wheel and you wont feel it with the Dodge, get fairly good MPG with it`s 6 cylinder engine and well, It`s a Diesel! They get almost twice the mpg if you know how to drive it with a very light foot and taking advantage of the massive torque (manual shift).
Also the Cummins engine is able to use "alternative" fuels without problems and they should last a Million miles ( Mercedes` cars do that!) heres proof
http://reviews.ebay.com/1980s-Mercedes-Diesels-647-500-Miles-amp-Going-amp-Going_W0QQugidZ10000000003602287?ssPageName=BUYGD:CAT:-1:LISTINGS:3
I say this as I own NONE of these trucks, but work in the Construction Industry as a mechanic and I know whats out there.!
Some people have converted Ford and Chevy trucks with industrial diesel engines, this guy put a 4 cyl cummins engine and gets 26mpg with a Suburban!
http://www.youtube.com/watch?v=uV903Ai7Uhg