Frustrating Electrical Issue
 

Confirm the batteries are good first. ie. not a shorted battery.

next try to isolate each with the battery switchs= these can go bad too btw...

I've had the diodes ( or whatever they are) go bad in the alternator themselves and drain the battery. If the batteries are good and all the fused circuits are good then try disconnecting the alternator.
edit- you are familiar with the method of using a volt meter or small lightbulb and pulling fused to check the individual circuits, correct?

yes have done the volt meter check on all the circuit breakers and never found it, so everything seems to be pointing to a bad alternator

Try getting yourself an amp clamp, it will show any power movement.

You have to measure current draw, simple voltmeter is not enough....

Simple test lamp connected "in-line" will show draw.

I gave you some direction on FB. Here is a step by step (although I didn't read it thoroughly).
https://www.wikihow.com/Find-a-Parasitic-Battery-Drain

Here is the cable adapter I referenced. You can find them cheaper.

a volt / amp meter is all you need.

Throwing this out as well

Stereo ? unhook all that power to it including amps

Automatic bilge pump --- the float switch possible stuck up so that means it runs all the time.,., Not the manual side of the bilge pump but the automatic side /. float side - have seen this alot thru out

any electronics / graphs / depth finders and so forth ?

any lites left on in compartments or so forth ?

Do you have battery selector switches that powers everything off ?

Alternator