I would go with an older Dodge mid 90's diesel. I had a 96 Dodge 2500 with a 8L V10 (10 mpg) and it was built like a tank, very reliable. My dad made me get rid of it because of the bad mileage.
Now I have a 2002 Dodge 1500 with a 4.7L V8 (11.8 mpg) and it is the biggest POS I have owned. I had to replace the tie rod, ball joint, and wheel bearing on the passenger side. It has a new PCM because a $60 vapor pump shorted out. I've had to replace both radiator hoses and the radiator. The bed is starting to rust out on both wheel wells. I replaced the exhaust that rusted through on the one side. It has turned into a money pit.
My friend had an old (85 I think) Ford F350 diesel crew cab dually that was ok on mileage. If you change the gear ratio in the rear you can see some good highway numbers. His topped out at 70 mph going down hill but he had an old 4 speed manual with no overdrive. It did well in snow and mud even though it was only rear wheel drive, all because it was a dually.
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