Only 24 volt stuff runs off bat 1 and bat 2 in series, which on the CUCVs will only be the glow plugs (which have a resistor to reduce 24 volts to 12 volts...but that doesn't really matter for this situation), starter, and blackout lights (and military radios, the NATO slave cable, and some other "accessories", which I'm assuming you don't have). Those shouldn't be drawing any power except when you are starting the engine or when you are running the black outs. So I would suspect the bat 2 isn't holding a good charge, so it is overtasking the gen 2 circuit and causing the light to come on. In fact, whenever a gen goes bad the bat is usually soon to follow, and vice versa. This is because when one goes bad it puts an increased strain on the other.
Because of the grounded gen 1 circuit which has a battery in series with the isolated gen 2 circuit battery, it will cause battery testers to give bogus readings. I recommend disconnecting each battery from the truck before trying to connect a tester. If the battery is good, I would check all the wiring on the isolated circuit to make sure connections are good, nothing is corroded, and nothing is grounding to the vehicle body or something like that. The gen 2 circuit is supposed to be completely electrically isolated. The only time electricity is used from the gen 2 circuit is when bat 1 and bat 2 are used in series to create 24 volts.
Both bats need to be whompers as far as cranking amps. If you have some wussy regular car batter, that might also be the problem.
So the NATO slave cable socket is connected to the two batteries in series. And sometimes there are other power cables (for radios and other accessories which are probably not on your truck) which are also connected to the two batteries in series. It might be worth it to disconnect those kinds of things, just in case they are allowing a small amount of electricity to ground to the vehicle body. These would not be in full short, or there would be serious problems. But on a 30 year old truck, it is always possible electricity is bleeding through worn insulation or corroded connectors or something like that.
Good luck.
Utah CUCV