If you read 13.8 volts at the alternator, that's low, but at idle speed it might be normal. Many alternators
just don't pout out useful voltage at idle speed. With the engine in neutral, at "crusing rpm" you should read 14.3-14.4 volts at the alternator, and the same at the battery posts.
If you don't read 14.4 at the alternator, it probably has a defective internal regulator or defective diode. Your choice, take it in, replace it, repair
the alternator shows 14.4 but you aren't getting that at the batteries, you'll need to trace out the wiring (charge and charge sense leads and ground) to see where the problem is.
In order to do that clearly, it helps to draw out (pencil!) a schematic that shows every "box" and wire along the way, including any a/b switch, any 'zapstop" or other gizmo installed, and so on.
You could just have some bad wiring, but the only way to find out is by first testing the alternator/regulator, and then continuing to veroify every inch of the way back to the batteries.
Of course if you are lucky, it is just an old worn glazed alternator belt (any real shiny spots or piles of black dust?) or the alternator hasn't been tensioned enough, so it is just slipping. First check the belt, and if it is 5 years old replace it regardless. If you can, get a belt tension meter to set the tension. Doing it by feel or by ruler works--but not quite as well, and too little tension lets the alternator slip, while too much can ruin bearings. So the tension guage is a "buy it once, use it for life" investment.