That is the way alternators go out, just all of a sudden. If you had thought to test volts while running with meter, that would have let you know if it was bad. When alt goes out, truck will run till battery gets low. Truck will run without alt connected till power depletes. Newer vehicles with computers will not run without alt connected.
The first time I met my now wife, she and her sister came out and we drove all over the Sierra Nevadas and the Inyo's. I kept seeing the check engine light come on, then go off, so I thought nothing of it. Our last stop was a hairy two lane stretch of road cut into the side of a mountain, so it was wall on one side and hundreds of feet to the bottom on the other side. It didn't help that it was partly icy. So after white-knuckling the 20-point turn without falling over the edge to our demise, we hiked around. It was 9 degrees, we were the only ones out there, no cell service, and the vehicle started up just fine. Sun was dropping behind the mountains as were starting back down. About halfway down I turned on the headlights and noticed they seemed dim. But with it being dusk it was difficult to tell. Going through town same thing because of the street lights. Leaving town I went to set the cruise control and the tach was at zero, the speedometer was at zero, and I said them "UH-OH", this isn't good. I think we may be make it though." My wife is quite adventerous so she wasn't worried. We tailed cars in front and soon the lights were getting dimmer by the minute. Halfway to our destination the vehicle quit so we coasted off the highway. Found out AAA doesn't have battery replacement service in that area. Got towed to the hotel, and got stiffed the next day with an overpriced battery from the only auto parts store in town. Even with the new battery I barely made it back home because they alternator had failed.
As honky said, take them both in and have both tested.