This will be an interesting thread to follow, I have a a horse transport that was based on the dodge trucks, so hopefully I'll be able to pick up a few hints.
This is my old girl
I get about 12,5 l/100 km (19 US MPG), the figure changes a little with two fat ponies in the back, but not by much.
I haven't done anything to mine apart from maintainance, mechanical fan to electric and a rev counter. Oh and slow down I normally cruise now at about 50 mph.
I do have plans to change the gear box and axel oil from mineral to synthetic. Also very soon I need to pull the turbo off and get that re-sealed, not sure if it will do much, but it should keep the engine cleaner.