I have had the exact same experience for years; couldn't figure it out. Then I did some reading about chargers. I purchased a ProSport ProMariner charger and everything changed after that. My batteries used to last 3-5 hrs and be done; even after "fully" charging them. Now, I can't get them to run down. I'm now a firm believer you cannot just charge a deep cycle battery with any old charger. An onboard marine deep cycle charger, like the one I bought, has 3 different charging cycles. And for the naysayers, I too thought it was a marketing gimmick but that conditioning cycle it goes through is something that a regular automotive charger, or a maintainer does not do; and it's made a night and day difference in the state of charge I now get.
So, what kind of charger are you using to charge them with?
Also, are you testing specific gravity with a hydrometer to test the state of charge or are you just using a voltmeter/load tester?