Skip, the average medium or low cost deep cycle batteries do lose 10
to 20% of their capacity per year. Even if batteries are kept fully
charged with the poor conditions on your boat its no wonder they
appear to have lost more than 60% of their original capacity. A
battery bank that is fully charged will not cause belt damage you are
experiencing unless; The voltage regulator is set to high, Belt
contact at alternator pulley is less than 120 degrees, or there is a
short in one battery of house battery bank. I will repeat, loss of
battery capacity alone will not cause drive belt damage. Using a
separate voltmeter at battery bank set voltage regulator to a maximum
of 13.8 volts until this problem is corrected. The first clue that
something was wrong in your power grid was your charger's melt down.
An onboard fire or explosion may be next.

I watch your posts weekly I guess for the same reason I went to
Saturday afternoon movies as a kid, to see the short subject
cliffhanger, Will the villain get lose from track before train kills
him?