[HO455]

Having only done this kind of checking on tugboats I'm not sure as what to expect as normal or excessive on a motor vehicle.
Did you check it with the motor running?
With the key on and then off?
If the voltage is affected by the key the go through and pull fuses one by one until you find the culprit. Then you will have to locate the ground
I would also check for AC volts. If it is present then the alternator is the most likely culprit.
I would talk to a semi-truck radiator shop and see what they find acceptable. For the most part electrolysis damage occurs over time periods (Unless you have the only steel boat in a marina full of plastic boats) and a long haul truck would be susceptible to damage.

Good luck.