GasGas and Chris Arbor are both making good points. A solo tractor unit has a heavy engine putting loads of weight on the front axle and you’ll only have a virtually empty box on the rear where the drive is. Traction suspension braking etc will be all wrong.
Look at the 4x4 Renault fire trucks used by the French for forest fires?
Any truck will be expensive if anything breaks, or is worn. Much more so than a big car/van based vehicle. Fuel economy worries will fade when you look at other costs.
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