I work at a marina and also, personally, have a Sea Ray with the ProMariner installed (along with being a technical advisor on a VERY LARGE Sea Ray forum). Also have many friends and family members with them. All I mean by this is that I'm not just giving you "heresay" advice or advice based on limited experience.
If something's causing the breaker to trip, then there's either an issue with the charger, the wiring or possibly even as simple as an old, tired breaker. If it's the charger's "time", well... it has served a good life - after all, it's 15 years old. If everything is operating correctly, a quality "built-in" charger will give you many, many years of service and having it charging all the time will certainly increase the battery life to what you mentioned above. I've gotten that many years out of batteries and many, many others. I'm not saying the little ones are bad - far from it - I own a few Battery Minders, myself. But to say that a quality built-in is a bad thing - there's something else going on. The ProMariner's have come stock on Sea Rays for years and they have had an excellent track record.
I'm not trying to convince you one way or the other - just offering some solid, experienced advice.