Help! Truck on side of road. Voltage steadily dropped to 7.5 volts and died

[DVT873]

Would be a good idea to replace the belt and the tensioner. Serpentine belts do wear and will slip. Most of the time they don't make noise. The tensioners get weak with age. If the belt is bouncing with engine running it is definately bad. IF it is slipping it can heat damage the bearing at the front of the alternator.
Fun fact. On ProStars slippage will heat up the coil on the air conditioning clutch. That changes the coil resistance which changes the amperage going to it. The body computer watches that closely and it has to be inside a narrow range. If the amps get outside the range the body computer shuts off the clutch. It will leave a code which reads something like "AC clutch over/under current". As soon as it cools down it will work again. For a while. Not knowing this or what the code means can make you lose your mind trying to figure out whats wrong.I've seen a lot of parts, including body computers thrown at that problem.
I do not know for a fact but suspect that most trucks with a body computer work in a similiar fashion.