Denny,
Your vehicle was equipped with an alternator that will run all of your accessories, and provide a charge to your battery, when you are in drive and have your foot on the brake (2hrs, 3hrs in traffic. I doesn't make a difference). The alternator should supply a sufficient charge.
If your electrical stuff dims or slows-down a bit, when you are at a stop, it's normal. There can be a problem if this stuff slows-down too much.
If you are in doubt, have your battery and charging system load tested.
An old battery, that is in a constant state of discharge, can take-out your alternator. The alternator will work harder to replace the amps in the battery. Your car's battery is only for starting the car. After your car is running, it's up to the alternator to maintain a good charge to the vehicle and battery.
And yes, your alternator is rpm dependant. It will put out more amps at a higher rpm. When your mechanic dose a load test, he dose it at 1500-2000 rpm.
The volt meter in dashboards today is a toy. It gauges very little. The cars of the 50's and 60's, had an ammeter. When the needle went into "-", you had a discharge problem. When the needle was in the "+", everything was good. It's not the volts that matter most of the time, it's the amps.
The only time that volts matter is, so that your engine's computer can continue to function. Most engine/powertrain computers, shut-down at about
10.5-10.9 volts. Your engine will stop. It's not a matter of amps at this point, because all of the amps have been spent. You won't even be able to get the starter to click.
I hope that some of this information helps you, GMdude GM & ASE certified master tech.