WDH= Weight distribution hitch, not many boaties use them because they don't travel a lot ( some do ), most anly go to their local boat ramp and just as well, a lot of boat trailers are not roadworthy . All caravans use them because they do a lot of miles at highway speeds . If fitted correct they distribute the load from the towball to the front and rear suspenssion so the vehicle sits level.
As for fuel useage on the LC200TTD in city driving , I don't use it in the city , I have other vehicles to do that , mine is used as a tow truck and going away vehicle. Reports vary from a best of about 12/100 to 15/100 , i guess it depends on the traffic and how you like to take off from the lights , if you floor it all the time it will use the fuel as you are using all the ponies and you have to shift 3 ton.
Its not just modern TD's that are better as far as fuel consumption goes, all diesels are better than petrol for 2 reasons . First compression engines are more efficient and second , a lt of diesel has about 20% more energy than a lt of petrol. The heavier the load the more evident this is, thats the reason you don't see any petrol trucks .