Thanks, this is what I expected in how it works, generally, and the test was covered by Jehines in a prior post, I believe. I wished the manuf. was more specific to explain changes when the charger is on, or via the generator, etc. Their legal dept. probably limits how much they can say.
The long lengths of wire do not apply in your case. The lengths in our boats are very short to begin with, but could be teh equivalnet of a bad/loose joint.
Nonetheless, if you are confident and have a mulitimeter and can measure the voltage as indicated, then fine.
You can alternately google "outlet tester", and pick one up and plug it into any outlet.
I was thinking about the charger scenario, the charger output shouldn't matter on the pos & neg how the input is connected polarity wise on the AC - should it be miswired (it matter for safety reasons however). If the charger is defective and emitting too much AC riding on the DC, and with the DC ground connected to AC ground / neutral, who knows what that can do ? And/or the charger may not like generator power quality.
You say when the charger is on, the light gets brigther, lift the charger dc leads off the batteries, then turn on the charger breaker and see what the light does. Tape the leads so they contact nothing when disconnected.