It sounds to me like you do not have adequate charging power for your battery bank. The VERY BASIC rule of thumb is that your alternator should be rated to about 25% of your battery bank, i.e. you should be running a 65 amp alternator to properly handle your 255 amp hour battery bank. This may explain why your alternator is dying and your batteries aren't charging.
I also think a 65-70 amp alternator would put a pretty heavy load on your (nominally) 7hp engine. If you go that route -- and I don't recommend that you do - be sure to incorporate a switch that can shut down the alternator when you need to put all the power to the prop.
My suggestion would be to both increase the size of your alternator and reduce the size of your battery bank. Maybe bump up to at most a 45 amp alternator and downsize the bank to two Grp 27s or even Grp 31s if you can boost the charging via solar.
This should still be enough reserve capacity for the kind of loads you describe, and should allow the batteries to get fully charged. Two fully charged batteries are much better than three anemic/undercharged batteries.
Let's hear what other folks think...
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT