Well, Croft resolved my electrical issue for me through some of my own Googling.
Turns out I could have solved this problem myself on Thursday evening in about 10 seconds without even going outside.
The problem I was experiencing is that none of my 12V appliances--furnace blower, fridge, lights, or water pump--were working. The battery was good, but no power was going out. Everything had worked fine until late afternoon on Thursday. I couldn't fathom what had happened in that time to kill my electrical system.
The first thing I suspected was a loose connection. I thought that maybe I hadn't wrenched the main connector enough because I didn't have much room to work. So, I went out and bought a teeny wrench. It didn't help.
I then went and bought a battery terminal cleaning tool and some special gel. They didn't help.
We blamed the cables. We blamed the converter. I ran I don't know how many tests with my multimeter. I triple checked my fuses. I quadruple checked my breakers.
Then I did some Googling and found reference to a battery turn off switch.
Croft asked me if that switch could possibly be the great big yellow button on the dash board I've referred to a few times.
Indeed. When it's pulled out, the 12V system works fine. When it gets hit by someone's knee because it's at a silly height and location, it shuts off the 12V system.
WHO DESIGNS THESE THINGS?!
I'm choosing to laugh even though I am profoundly embarrassed.
But the upshot of this is that my battery job was good!
"Life's but a walking shadow, a poor player that struts and frets his hour upon the stage and then is heard no more. It is a tale told by an idiot, full of sound and fury, signifying nothing."