[v8volvo]

Did the tachometer reading also drop when the squealing occurs? Usually when the alternator belt is slipping you can see it on the tach, since the tach reads RPM off the alternator.

Now that the squealing is gone, but you are still noticing the voltmeter reading low when the electrical load is high, does the tach still stay steady? If you are seeing big variations in your voltmeter reading but the tach is reading normally, then the belt is probably tight enough now and the voltmeter is probably just a little oversensitive given the range of its fluctuation. Well, either that or the voltage regulator is acting up and/or brushes are worn.... You might want to get access to a known accurate voltmeter and check charging voltage at the battery with engine idling and electrical accessories, lights etc turned on to ensure that the alternator output is sufficient and correct, but it sounds like you have probably cured it.