Mind you this was a tractor versus a truck but the basic issue was the same. The old gal would just act as if the battery was non-existant and clamping on the jumper cables along let it fire right up.
It was 2 things to make it more fun. First the battery terminals and posts needed the usual wire bush and tightened up. But then it did it again. This time I lucked out. After a little poking and prodding I got lucky. I was standing back as Meryl gave it a try and I spotted a spark from the cable that ran down to the frame. AHA!
Off came that connection to the frame for the wire brush plus some terminal grease after it was clean. Bolt it back down and it's solid.
Got in my truck to start it the other morning and no power at all to anything. next day went out in the afternoon, hooked up jumper cables to jump but I didn't need to. It started on first turn and seemed to have full power. Let it sit a day and it starts no problem. Seems to have full power.
We had some problems 3 months ago and replaced the battery and had the alternator checked and they put a gauge on and said the alternator was OK, not the best but OK.