Duramax Alternator

[RI Chevy Silveradoman]

I was having issues with batteries being dead. I fully charged both batteries and truck was fine. After running the truck, then letting it sit overnight, no start the next morning. I fully charged batteries again and got truck fired up. I put volt meter on battery and got 13 volts. Lat time I checked I was getting around 14.7 volts. Belt is good. Alternator is original.
I spoke to someone who was telling me there are 2 charging circuits for GM, low speed and high speed charging based on engine RPMs. Has anyone heard or know about this? Anything else I should check before getting a new alternator?

 

[ak diesel driver]

If the batteries are really low or internally shorted a tad it could bring down the voltage