I noticed the red battery indicator light flashed on when I was driving the other day, and has not happened again. I'm wondering if this is a early indication of alternator problem. The truck is still all original parts with about 70,000 miles.

What is the condition of the drive belt? Was it possibly wet when the light was flashing?
You say the light is off now but have you tested the starting and charging systems?

With the engine running, check the regulated battery voltage--it should be 14 to 15 volts. If it's less than that, replace the voltage regulator/brush assembly ($20) on the back of the alternator. Be sure to remove and fully charge both batteries, plus clean the battery terminals and cable connections. While the negative battery cable is removed, remove and clean the cable connector going to the alternator.