I have no experience with diesel, but we have a 96 NPR 5.7 gas and a 2005 NPR HD 6.0 gas. Both are 14' stakebody flatbed dump trucks we use for hauling bulk material, broken concrete, sod, etc. They seem to get about 7.5-8 mpg in town. I drive these trucks on the highway so rarely I don't even have a gas mileage number. Our typical loads are very close to GVWR, so around 6000 for the HD and a little less for the regular NPR.
While the MPG isn't terrific, compared to my 1 ton chevys, its close. The 99's get around 9-10, and my 2011s get about 10-10.5 unloaded or towing 10k. Maneuverability with the isuzus is stellar. Also, for what it is worth, the newer models have 6 spd transmissions, which may help with mpg some. Both of mine have 4 speeds.