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You didn't say anything about 6000 liters. I'm willing to bet a 416 would do it. Put it on a trailer and tow it. A 416 would still be cheaper than modifying a 5 ton to go that slow :)
No way Brits would come up with something this well designed and as reliable as these are :)
http://www.emercedesbenz.com/autos/mercedes-benz/unimog/mercedes-benz-history-an-in-depth-look-at-the-mercedes-unimog/