Assuming that you are NOT on shore power, and that you unplugged at least an hour ago, when the switch is in the Use position, what does your monitor panel say your battery voltage is?
Sometimes the relays inside the switch stick in the off or on position, so you have to click it on and off a few times to get it to switch over.
Your monitor panel should be giving you some voltage reading, and it will be helpful to know what it is right now, assuming you have been off shore power for at least an hour. Let's start with a new reading, and not rely on what was discussed in the past. In Post #22 you said the battery was totally dead. So the monitor panel is also totally dead?
Even after you try clicking the Use/Store switch on/off a few times?
PS -- I think that the red light can be on when you are plugged into shore power, even if you have pushed the bottom of the switch to "Store" which will result in no battery charging, even though the red light is on.
Is it possible that you were not charging the batteries because you assumed the red light meant that you were charging them?
I would also check all the connections at the battery posts. You possibly have a loose connection there, or where the 12 volt
battery power comes into your 12 volt
distribution panel. A reading of Zero volts would be quite unusual IMO. As others have suggested, an hand-held voltage tester used right at the batteries, would confirm the voltage there.