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I'm not always good at explaining stuff, and have had a couple rum & cokes, but: Both car and bike systems share some similarities. They both generate AC current that's converted to DC and maintained at a regulated voltage. With an alternator the rectifier (aka diode trio) and regulator are separate parts and usually housed within the alternator where on a bike the rectifier and regulator are usually one part and always separate from the stator. In a car's alternator a set of wire windings are spun within stationary magnets, with a bike's stator the magnet is spun around the stationary windings; different action with same reaction, AC current is created.
A car alternator's output is regulated by varying the strength of the electromagnetic field to make it work as hard as needed to maintain system voltage. When more power is needed the alternator works harder to put out that power. This can be witnessed by turning on the head lights or other stuff and hearing the engine sound change. With old non-electonic controlled engines the RPM will drop, with modern stuff the idle control system opens the throttle to maintain the RPM, either is generally audible. If the battery is at a full charge state the alternator is pretty much free-wheeling doing no work.
A bike's stator, assuming it has the usual shunt type regulator, is always putting out as much as it can. The extra power not needed by the battery/electrical system is consumed by the regulator, it's basically a controlled short circuit and is why they tend to get hot. This can be witnessed by turning the headlight or other loads on & off and you'll note no change in engine load.
There are newer bike regulators called series regulator that actually lessen the load on the stator rather than just being a shunt that consumes the excess power.
With your truck the alternator was maintaining the good voltage you were seeing so all appeared well. Problem is that it was possibly working like mad because half of the batteries were sucking up a lot of power to maintain their voltage. Hard to say without knowing what the actual failure of those bad batteries was, but the fact that removing them put an end to the alternator failures is as close to proof as you can get.
And I gotta know; WTF 4 starting batteries?!?
Edit: BTW; some bikes, like my old Motto Guzzi, BMW, and some others use an alternator just like a car but the usual is a stator system like the CBs use.
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