1996 GMC Jimmy 96 Jimmy not charging properly.

96 jimmy s15 v6 auto. I noticed last week that when I pressed the gas the voltmeter on the dash went down and headlights dimmed. All returned to normal when I let off of the gas. I had the battery checked and needed replacing. I had the alt bench tested 2x both tests showed good (diff reading both times on same machine). I had the system checked in the truck while running and said I needed an alternator. Replaced that. Now truck charges at about 10-11 volts according to guage. With accesories on put a large drain on the truck. There are new battery terminals, alternator and battery. Any ideas here? These tests and results are getting expensive.