[denverpilot]

Thought: You said this started when 120 VAC high current loads were browning out the whole local 120V system.

Have you checked to see if your battery is even charging? Perhaps the voltage spikes and brownout damaged your converter. (Which charges your battery and also provides plenty of amps of 12VDC to the lighting and other 12VDC accessories.)

I'm suspicious that maybe you're just running off of battery power and the converter is gone, the AC feeding the converter isn't getting there, or similar.

There aren't too many heavy power draws on the 12VDC bus, other than the furnace fan, in most setups... So if you have only been using the lights that require 12VDC you may be just slowly discharging your battery, and nothing it charging it.

You said you checked breakers and fuses, but I'd check again. Or see if the 12 VDC system voltage is slowly dropping over time. With AC powering the converter, when it's in charging mode you should see something above 12V anywhere 12VDC is used. If you're seeing less than 12V, start hunting for why the converter isn't charging the battery.

Just a thought.