battery problem

I have a 99 Suburban that has been having a battery problem. If I leave the battery connected it sometimes discharges enough to need to be jumped. I
normally use the quick disconnect I put on it every time I shut it down for any length of time. It was so bad yesterday that I needed to jump it 3 times. I took it to Walmart and the battery was bad, the 3rd one in 2 years. Is it possible that the problem is all in the battery?
Related question, I would like to set up a dual battery system. Instead of a battery isolator, could I just put in 2 power diodes in the charging cables?
Mike D.
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