[HO455]

I know that pure distilled water is less of a conductor of electricity than any less pure water. The lack of ions and other contaminants make it less conducive.
I looked at your link and it is on a site that sells electrolysis control products. I am always suspicious of information presented this way. They may be setting a low bar of acceptable voltage. I would do some further research from more real world sources that may not be profiting from the information presented on their web site.
How are you using the truck? The .4 volt limit may be unsatisfactory for a long haul truck but not necessarily a problem on a truck that only sees 5000 miles a year.
I am quite sure that the limit can be variable depending on usage. I know if I found the voltage you found on one of our tugboats I wouldn't spend much time chasing it down. That low of voltage to would be difficult to isolate on a boat. We would expect the Nalcool and zincs to protect the coolant system.
Electrolysis can also be cause by the water flowing over dissimilar metals. Having iron, steel, aluminum, brass, copper and others in contact with the water can create current flow in the water. This is why most quality aluminum radiators these days have a provision to install a zinc in the coolant. It is a sacrificial component to keep the rest of the components in the system from eroding or corroding.
If you install a zinc for protection then it needs to be checked as per the manufacturer's schedule as once the zinc is eroded the protection is gone.
As always these are my opinions and should be treated as such.