My F350 6.7 SRW diesel got 12.2 mpg average over 3200 miles pulling a 40 ft 16500 GVW 5th wheel. I have got as high as 21 mpg empty and some times 18 mpg. It depends on the Ctane rating. B20 cuts 1 mpg while B5 increase the mpg because it actually increases the Ctane rating over 45 While B20 decrease Btu's too much. Some states like California require a Ctane of 50 which is the same as Europe. However most states allow a Ctane of 40. A Ctane of 50 will provide the best fuel mileage as well as better lubrication of the injectors.
Diesel trucks are better suited of heavy loads and some 5th wheel trailers demand a diesel truck which provide the best performance as well a towing capacity.