Batteries can develop internal shorts or at least electrical leakage and run a cell or two down.
Also try this: Charge the battery. Then disconnect the ground clamp & cable from the battery post at night. See if it still has a full charge in the morning.
Then there's the possibility that you have a short somewhere else.
Notice as you disconnect the cable, whether there's much of a spark. In the old days, there would be no spark at all if everything was OK, but now there are things like digital clocks, alarm systems, etc. that draw a minor current. If you are seeing significant current with everything shut off, you can try removing fuses one by one until you eliminate the current.
That said, I'm amazed that your battery lasted 9 years. I would just go ahead and replace it.
Ditto on using distilled water.
By the way, I had terrible luck with Exide batteries. Went through three of them in about 5 years with my truck, the last one being a "premium" model with a looong warranty. They all pretty much acted like yours, and would NOT hold a charge.
I replaced it with an AC Delco battery which lasted as long as I had the truck.