Ground points exist all over creation under the hood. Most any metal bracket, brace or bar is a ground.
TO FIND ONE you need a 12 volt test light.. You find this in the ELECTRICAL wall at most any automotive department or parts store. Looks a bit like an old fashion ICE pick save it has a clear or colored handle with a light in it and a lead with a clip.
FIrst make sure the light works (I often have to reseat the lamp) by testing across the battery terminals.
Then find a known 12 volt point (LIke one side of the isolator relay) and hook the clip
Touch the tip to most anything. Good light = good ground.
Drill a hole and insert a self tapping screw.
One example of a good ground on a Workhorse is the brake bell crank bracket..
Just make sure you do NOT hook to or obstruct any moving parts.