I think the battery might have a dead or dying cell. We went through something similar with my crew cab on Saturday. Got up early, breakfast at Whataburger (SOP for early gigs) and then heading across Houston to a toy show. Running the lights, stereo and heater all the way over there (cool front came in so it wasn't beastly hot for once!). No issues, gauge on 13-14V. Went to get the truck a few hours later to go load out, nothing but clicking. Got a jump and headed to the nearest location of the parts chain the battery came from last year. They checked it, sure enough, dead cell. Truck starts quicker than it has been, too.....
Earlier last week, we were setting and breaking high temperature records, then midweek a cold front came in and we went from 80s at night to low 50s. And old batteries don't do well with wide temperature swings....
We used to notice at the parts store, that it was quite rare to get more than 3 years out of a battery. We only handled good brands and charged every unit before it left the store. We always ran the highest CCA battery that would fit in the tray, checked the water level, and kept the terminals clean. This is in Houston, Texas where it's always hot and humid in the spring, summer and autumn, and can get fairly cold in the winter.