Welcome to the party!
We have a recently new to us 2002 Sailfish that had battery charge retention and charging issues when attached to shore power. A digital voltage multi-meter is worth the small investment and keep it handy on the boat. If any part of this is out of your hand-i-ness comfort zone, hire a marine electrician for an hour or two of work, it's worth the peace of mind. We hired one for 30 minutes to check my work when I was done.
With at least one battery connected to the negative leads, and the leads from your battery charger well separated, you want to measure the voltage from the contact from the battery leads to the positive pole on the battery. What we found on our boat (two bank charging leads) was that one of the inline 30 amp fuse holders coming off the battery charger was completely rotted out. The lead read nothing from the battery terminal attachment point, but we could get a good voltage reading from the inside of the battery side of the fuse holder. Bit of chasing to find it, but we replaced the fuse holder and made up new pig-tails for the charging leads, and made sure our battery attachment arrangement matched the factory specs in the owners manual.
So far we haven't managed to damage the batteries too badly, being new to power boating and having the occasional battery switch forgetfulness.
Also would recommend a fresh set of o-rings for your fuel filler caps. There's a flat area above the threads on the filler caps for fuel and water that's meant for an O-ring. If the o-ring is dry, cracked, or missing, it affords water the opportunity to make it's way into your fuel or fresh water tanks.
But seriously, the battery bank on your boat is worth spending a few bucks on an electrician to make sure you're in good shape charging wise, you wouldn't want to cause damage to other systems or cook your batteries!
Bob.
