Hi ben g
The problem is most probably the power unit, either one of the power devices has failed (thyristors or capacitors) or more likely the logic in the power unit has failed. The reason why you are getting battery voltage on both sides of the direction contactors is that the contactors have a common positive from the key switch and a negative for the contactors comes from a safety relay in the power unit logic. The relay supplies a negative feed to the foot pedal micro switch from there to the direction switch and then to the forward or reverse coil. If the truck faults the relay opens and removes the negative feed, so you will measure battery voltage on both sides of the direction coils.
Shane