Over the past 10 years, I seem to remember that I have found several ground bars in nooks and crannies on our old rig.
If you are trying to find a loose ground that pertains to a certain circuit the offending connection could be near that circuit's ground bar.
I do not know, but I suspect there could be one central primary ground bar to which the peripheral bars connect.
On our rig the chassis bar could be in the basement compartment below the driver or the domestic in the 12v utility bay in the basement under the passenger seat.
The schematics for our rig indicate only a that connection to ground exists, not where it is connected.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs