try pulling the battery, get a jumper extension harness for the battery, connect it to the truck while the battery is out of the machine and then see what you get.
In most electric's i've worked on, especially older units with older batteries the batteries tend to get acid between the cell's and can cause a bleed effect from the case of the battery to the frame and it back feeds through the motors into the electrical system. If your motors are isolated well from the frame then check the standoff isolators on the power cables, there are some in the main battery cable circuit to the panel, i have seen those go bad on occasion.
Most of the time you may not even notice anything wrong (except if you have accessories attached, they do not have the capability to handle any huge differences caused by this which may be why your having issues).
Once you get the battery removed, if the voltage dissappears theres your answer. If it does not then go by what crown said and start disconnecting components till it goes away.