There are 6 cells in a battery. It only takes one to short out, overheat, and expel the rotten egg smell (?Hydrogen sulfide?). Once it’s done expelling it, that cell was dead, no more charge, no more smell.
Most trucks really only need 2 barriers to start in the summer so you didn’t notice the lack in start power. A voltage check should have shown one battery at 10.6V or lower.
A month ago the first cell went, you recently had the next cell go. Hopefully they where in the same battery!!