"...Also, rpm can affect belt slippage. At idle, the belts might be okay, but when the rpm is higher (when driving the car at speed), they may start to slip..."

I had that very situation once. The alternator was spinning but not nearly fast enough. The belt was slipping but with NO squealing. It was late at night and the only way the fuel pump etc would keep running was to turn off the headlights (parking lights and tail lights on) and tailgate the car in front of me into the next town.

What happens to your voltage measurement when you really load up the electrical system - headlights on high beam, heater blower on high, rear windscreen demist on, radio, etc.? What does it do when you rev the RPMs under these conditions?