Voltage problems (I think)

[brucehugh]

I have a 1988 30' Luhrs Alura with a single Chrysler v-8 engine. Last year I had to call the towboat guys when the engine refused to start (wouldn't crank). The towboat gave me a jump start and away we went toward the dock, only to quit again 500 yds from shore. Dead battery again even after running at 3000rpm for half an hour. I suspected the alternator so hauled it off to the local electrical shop where they tested it and said it was fine, putting out as it should. I later found out that my batteries (2 Sears deep-cycle) were almost totally dry. That was last year and this is this year. I have two new batteries, but when running yesterday to fish, I watched the voltmeter alot. It only registers about 10 volts, seldom higher even after charging for several days. It began to register about 9 volts after running out to fish and then dropped to about 6 volts after fishing for a bit. It started ok several times when fishing but even after running back to the dock at 2800rpm for almost an hour, it still read about 6 volts. I think it is about to do the same number as last year so I don't think new batteries and the checked-out alternator have solved the problem. Where should I start? BG