Re: Thomas Bus normal alternator output
Obviously your dash voltmeter is reading low since your multimeter shows higher on the batteries. Did you check the voltage at the batteries at high idle, say 1500 RPM? Alternators need to spin to make power and it's not uncommon for vehicle to discharge when at idle. That's part of the reason why modern cars have PCM controlled idiot lights...they don't want the charge light coming on when you have your Grand Am stopped at a red light in gear with the heat on high, the stereo blasting, the rear defroster on, the headlights on, etc.
You could have a faulty alternator, but your symptoms sound a bit like dirty and corroded connections. Check the output of the alternator by going on the big positive stud on the back of it and touching the other meter lead to the alternator case. I bet it will read higher yet in which case I'd be looking at cleaning the battery posts and maybe replacing some cable ends.