Alternator

If you are changing the alternator and the charging is above 12 volts, then after a few days it drops then we either have an issue the wiring or the alternators you are getting are just not any good.

The way to test this is to load test the alternator when it drops below 12 volts.

Also, we need to load test the battery. Just because the alternator is supposed to maintain the charging voltage, if the battery is drained then the alternator will not be able to properly charge the system.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

https://www.2carpros.com/articles/car-battery-load-test

Once we know the state of the battery and alternator, we can move on, but this is where we need to start.

Please run through these tests and let us know what you find. There is a chance depending on what the voltage is dropping to that this is normal. Basically, the voltage will not always be above 12 volts. The alternator can do what is called duty cycle which means it can raise and lower the voltage depending on the load.