Well the 1 electrician customer I have that has owed me money since August stopped by to pay today so I told him he had to answer this question as interest..
He said my drawing is correct but I need to add a wire from the neutral bar to the connection of the green ground wire to the box..
That is not correct. ... You only need to connect the neutral to the equip ground at the main service panel where the utility Co entrance is. This is not the case on your truck.
Think about it. Your incoming wires are red, black, white (neutral), & green (ground). If the white (neutral) & green (ground) are cross connected then one wire is useless & unnecessary. The white (neutral) is a current carrying wire. The green (ground) should not have current though it except in a short to ground. These wires should not cross connect in your box or any place between your box and the main service panel.
At the main service panel the ground bar (green) is directly connected to a ground rod into the earth. This is where the neutral connects to earth ground and returns to the utility Co. by cross connecting to the ground bar.
On your truck, you do not have a ground rod connection. This is the reason for the 4-wire shore cord, to provide separate paths (wire) for the neutral & ground back to the main service box.
Cross connecting these two wires in a sub-panel is a safety and a fire hazard.