My 2004 2500 diesel w/ 91K normally displays reasonable MPG. Headed to get fuel and display started decreasing...17.5->15.5->12… it settled down to about 11 mpg. Pressed reset. It jumped to 99.9 and then steadily decreased to 10 mpg range, where it remained for the 350 mile trip. Obviously something is haywire. I suspect sensor(s). What sensors contribute to MPG calculation? Will a diagnostic scan identify bad sensors or the cause of the problem?