Ethanol is highly corrosive. Without specially-designed lines, tanks, seals, and fuel pump diaphragms, trucks can really suffer from higher ethanol content in today's fuels.
TheDieselDriver has an article on the warranty and repair issues caused by varying ethanol standards in America, today:
Five years ago, biodiesel was a form of fuel that the diesel enthusiast would convert his or her vehicle to use. Since then, it has been forced into the mainstream through various government mandates and subsidies, in the form of blended fuels that are often the only option for drivers of diesel vehicles. Although biodiesel has the potential to reduce both dependence on foreign oil and fossil fuels, its premature introduction into everyone’s fuel tanks, and especially the inconsistency of the biodiesel and the amount of it being mixed in are starting to cause problems.
Heavy truck operators, especially, have to know what's going into their tanks in order to properly configure their vehicles.