Hello I need help

My alternator is not charging. My first alternator died and blew the 175amp fuse. I replaced both and everything worked. 2 days later the battery light came on then turned off after I parked it for a wile. A week later battery light came on and stayed on, was thinking a defective alternator. Replaced the alternator and did not fix problem. Battery is good 12.6volts, all fuses are good, I am getting battery voltage on the field side of the alternator but when the truck runs I only get only 12.6 at the output of the alternator? There is no drain on the battery over night, and I have checked all of the fuses inside and they look good.

Any ideas would be very helpful.