Why Batteries should be Load Tested

This type of battery failure is pretty common in my experience.
The first indicator is often low State of Charge Voltage; the battery can be charged, but voltage won't hold above 12.5, and voltage drops rapidly with inactivity.
Most batteries exhibiting this condition will carry some load, but nowhere near the rated value. Slow cranking speed is often an indicator.
The only way to test objectively is with an adjustable carbon pile test tool like the one seen here, where amp load can be increased to the rated level of cranking amps.
These tools used to be hundreds of dollars, but are now available from Amazon or Harbor Freight for less than $100.