A need?... I would probably answer no.
But, that said.... take a look at getting a ScanGauge-D (scangauge.com). It handles the J1939 & J1708 ports that your diesel will have and is more than a scanner.
It lets you monitor parameters that are not on the dash as well as track things like MPG, engine load, HP, miles on tank of fuel....or of course the one everyone wants to know 0-60mph time.
This was the first thing I added, even before a good Nav.... $160ish.
oh, yeah... it also does diagostics and clears codes.
Doug & Kathy
2004 Fleetwood Revolution 40C