Sounds more like a loose or poor connection. When you say your greased the terminals, what did you use? Did you grease an then install the batter cables? Was there a bunch of corrosion prior to this failure?
Inspect all wiring very close. Check the main power wire to the fuse block (12mm on terminal and I think a 10mm inside the fuse block. A the next failure see if there is power in the fuse block with your volt meter (most fuses have a voltage tap on the back that you can touch to see if there is power).
A dead cell in a battery will not usually rear down to 0v. It is possible that you devolped a draw with the key off. Do you have any aftermarket radio equipment wired? Fridge? Does your volt meter have an amp setting?
2004 4runner just starting to build
83 Hilux SR5 is being sold
94 pickup in a pile of parts