Newbie With a 1983 G Series Van & 6.2

[Dvldog8793]

Howdy
The regular vans (any GM that uses two battery system for JUST vehicle operation) do not have an isolator. Typically an isolator is only used when one of two batteries is used for a completely different purpose, like running your camper. The isolator allows the battery to be charged and used without discharging the other battery. For vehicle operation(not camper or other stuff), the two batteries should be hooked directly in parallel.

Grounds can always be an issue and should be checked. Most of time the smaller grounding straps are not going to effect the starter. Your main ground cables to the block or frame could. Bad grounding straps or wires might be the problem for your low voltage charging or voltage gauge reading. I would hook up a multi-meter direct to the batteries and see what is really happening with voltage.
If you KNOW the cables/connections are good.... it sounds like a weak starter to me. As I posted earlier, we just had a similar problem. New batteries, new cables and grounds, good charging, the starter was not that old. truck did not like to start and would require 10-15 seconds of turning over before it would light. The starter never failed, but to my ears, never really sounded fast enough. We replaced the starter and problem solved.

Having a battery voltage of 12.4, when not running, is an indicator of healthy battery cells. Measured under load when starting your voltage shouldn't drop much below 10.5 or so.
ALL my vehicles have alternators, I have not seen a generator(all alternators can technically be called a generator) on a standard vehicle for a long time. Maybe your truck is different due to it being an RV type. Not saying this is FACT just that nothing I own or have worked on(besides my tractor) has a bonified generator.
IIRC....the term "alternator" was a Chrysler term, GM had delcotron generators and ford had AC generators. All of them were the same deal, meaning what we currently call an alternator, an internally rectified DC generator vs a dynamic DC generator.
"Alternator" has become the "crescent wrench" for internally rectified DC generator systems in vehicles. It did take GM longer to switch to an internally regulated system vs an external voltage regulator.
Again this is just what I remember from basic vehicle electric classes, and my memory is real good just kinda short....