Voltage is electrical pressure, if your batteries have a dead cell or two, they will resist charging. Think of this like a 100 gallon air compressor trying to bleed off all its volume through a tiny pinhole, its going to have a lot of airline pressure behind the pinhole as all the air trys to get out versus if it had a gaping hole in the reservoir, not much pressure there. Pressure is resistance to fluid flow in hydraulics, and if you have bad batteries or poor connections, that will resist the electrical flow into the batteries, making more electrical pressure, or voltage. Im not saying this is your complete issue, but I havent seen it mentioned in the thread before, and I think its worth mentioning as I see it cause all kinds of issues in these old trucks.