My Optima yellow top has a neat aluminum cover that RP built that incorporates a Flaming River cut-out switch. I had forgotten to throw the switch off after our last trip and the battery was dragged down so I charged it over night. Yesterday when I tried to start it the gauge read about 12 and there was a click when I hit the ignition, then the gauge dropped to zero and nothing. I Turned the Flaming River of and on and tried again, with the same results. Then I tried jumping from my Ranger to the battery with the same results. Then I hooked the jumper to the terminal going out of the Flaming River and she started right up. So would this indicate a bad switch or a croaked battery? The Optima is a 2012 so 7 years ain't bad, but they are pricey. Can it be checked at a parts house?

Sounds like a bad switch. Try jumping around the switch with a cable, or just temporarily bypass by hooking the output cable from the switch to the input side of the switch. If it starts then we know the switch is bad. If not the batt may be toast...

Yep , what Mr Steve said . Jump across the switch and give it a try . I've changed quite a few master switches on fire trucks over the years.

I brought the battery up to O's and it showed good, so I took off all the connections and cleaned them and hooked everything back up the way it was and Voila! Great cranking power, which I haven't had in about a year without the aux, and it fired right up. Back in bidness!

One more quick question. Isn't there some kind of grease for terminals to keep them clean and functioning well? I'd take all of it apart again and grease them up good if I thought it would save me a headache down the road.

I worked for years as a mechanic and what we found that works great is simple wheel bearing grease .
I've had really great luck with it over the years.
I put it on the battery posts then install the cables . Finish by coating it all down after install.

4X Fluid Film in a spraycan
https://www.fluid-film.com/
Put baking soda and water in a bucket, stick cables as far down as possible and soak to clean, about 15-20 minutes, you cannot see all corrosion, then apply fluid film.