I've since unlocked the bootloader, installed the latest TWRP, wiped data/factory reset, rooted, etc. Nothing has made any difference. The fact that everything was fully charged up seems to support the original owner's claims that she never had any issues with it. I just don't understand how it could be fine one minute and suddenly have issues once I take possession of it. I'm pretty anal about caring for my gadgets, so it didn't suffer any bumps or any kind of mishandling in the 24 hours that I've owned it. I'm at a loss. Any ideas?
The only activity I've seen from the dock's charging LED came after the dock was fully discharged. Now, if I have the tablet docked and the charger plugged into the dock, the LED blinks orange. I've never seen the LED do anything without the tablet docked. No idea if that's significant, but thought it was worth mentioning. I did some searching and found that some people had charging issues with their docks, but didn't stumble across anything that seems to match what's happening with mine.