Voltage is not a good measurement for battery (power cable) leakage to the frame of the truck, nor is positive to negative at the battery connector.
The correct answer and method to check depends on the controller manufacturer's spec, GE usually calls for no less than 100K ohm resistance between frame and any power cable (not just the battery cables).
I am pretty sure that if you can light a 12 volt test light between the battery post/cable and frame of truck, you have too much short, but reading the voltage only has the ability to clue you into which cell on the battery has the most short to the case, not really how much current that short may carry, nor if there may be some other short that is not showing at that moment, since the path of least resistance* at that point in time is the voltage short you are looking at
* the path of least resistance for a.001 watt current may not be the same for as for a 1000 watt current