Workhorse made a a couple diesel chassis. One if the R series, which is a pure diesel pusher. Another is the UFO, which is a rear engine chassis that can have either a gas or a diesel powerplant. And they currently offer a MaxxForce diesel in the W16, W20 and W22 front engine chassis.
This article on the 2007 Presidio says it comes on a Workhorse R chassis.
you can read about the R chassis on the Workhorsr site.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL