Low voltage

I have a 94 jeep wrangler i bought in March this year. It has been great until sept. My husband drove it to work and it died on the side of the road and we thought it was the alternator and changed it. That night driving it home it seemed fine but the voltage was low. The next morning we went out to start it and it would not start so we changed the batteries and it started up. Later that day is was driving and the volt meter dropped down and it died again. It has dual batteries in it and in order to get it home i switched the batteries and i made it home and it died again. Then i changed the dual battery isolator hoping that would work. A couple of days later we went 4x4ing and to died again. i have now changed the alternator twice and the dual battery isolator twice and it still is draining the batteries and not recharging them. Any ideas???

First place I would check would be the battery cables. If they're corroded on the inside (peel back insulation to check) then they could cause a voltage problem
--edit- corrosion is usually near to the positive battery terminal(s)--

Second place I'd look would be the batteries. If one of them is bad, it could be wrecking the whole system. A battery tester will tell the tale.
--edit- be sure to disconnect battery cables prior to testing--

Third, would be any voltage regulating equipment. I'm not familiar with a two battery set up; however I'd consider running a single battery and see if that clears up the problem.

Good luck with it...