When driving, lose all electrical - lights, guages, radio, A/C. Replaced both batteries on first occurrence... vehicle ran fine for 3 days. Now problem has returned. Lights/guages will flash briefly every 5-6 seconds. Eventually vehicle will stop running. Vehicle now will not start.
Is this an alternator issue or possibly something much worse?

Sounds like alternator same thing happened to my parents blazer u could go to as local u pull parts place and get one cheap hope this helped

Replace the alternator's regulator/brush holder assembly--the brushes wore down too far. Disconnect the negative battery cable, remove the 3 bolts to the back cover, and use a Torx 20 bit to remove the 3 screws to the regulator/brush holder assembly. 15 minute job.