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?
83 Hilux SR5