Hi duodeluxe;
What the concern should be is the amount of current leakage going through the truck by way of the truck's chassis. Basic electrics, for current to flow from a battery it must have a path from the positive plates to the negative plates. Using a volt meter to determine leakage will only tell you that there is a potential for current to flow and the likely source of the flow. GE in their SCR speed controllers state that current leakage of 100 mA or more is excessive. With the introduction of CAN networks and AC drive systems I've noticed that the tolerence level for current leakage as being less than half the 100 mA. In keeping with the thought of current leakage the source originates at the battery but wiring, insulation breakdown, dirt, dampness, etc. can be the route taken.
By now you are wonder how to determine if leakage is occurring and how much? With the battery in the truck and the battery's electrical connector disconnected from the truck. Using an amp meter placed on it's highest amp scale and the BV+ (POS) of the meter at the positive post of the battery and the BV- of the meter in contact with a cleaned surface of the truck and touching it's bare metal measure the amount of current flowing. Dialing down the amp scales of the meter to the lowest readable scale. IF YOU SEE 120 mA DO NOT GO TO THE 100 mA SCALE or LOWER. Current flow in the states in the industrial truck industry is positive to negative but the others say flow is from negative to positive for other applications. We have one half of the questioned answered. We now know what the flow of current out of the battery is. Now if we were to place the BV- (NEG) probe of the meter on truck battery's negative terminal and the BV+ on a clean bare metal surface of the truck and the amp scale read the amount of current flowing.