I would suggest that you venture over to this site:
Electrical Archives - Marine How To There is a vast amount of information there on many subjects and the author of the articles has a way of explaining things so they are easy to understand. I am a firm believer that understanding how things work is the first step when doing the sort of upgrades you speak of. I have learned this lesson many times and wasted a serious amount of money doing upgrades like this only to find out afterward that the route I chose was wrong and had to go back and do the research and redo the job. It is unfortunate that I did not spend the time to do the research first as my battery, alternator, and charging systems upgrade cost me nearly double what it should have.
I also agree with Don about parasitic loads. While it is complicated to track down these types of loads it is easy to check to see if they are present. A word of caution here, battery charging can produce explosive gases. Be sure the battery charger has been off for 15-20 minutes and there is good ventilation around the batteries before you do this. All that you need to do is turn shore power and every dc device in the boat off. Don't forget the fridge. You can then remove the negative/ground wire from the battery. When you do that you should not see or hear any arcing. If you then watch very closely as you touch the cable back on the battery post you should not see or hear an arc. If there is an arc you have a parasitic load that will drain your battery. Many boats have parasitic loads that are not realized during the boating season because the boat is plugged into shore power and the battery charger and boat use hides it from you. This is not uncommon. There is however a simple way to stop these loads from killing your batteries. If you can't find the load just charge the batteries fully and then disconnect them from the boat and each other. If the battery's state of health is good the batteries should hold their charge through the storage period. Doing this can also give you a good indicator of the health of your batteries. If the battery voltage after storage is between 12.2 - 12.8 volts your batteries are in good condition. If the voltage falls below 12.2 or lower the batteries are nearing the end of their useful life and should be replaced soon.
I realize I did not answer your question and that was on purpose. I don't know what your needs are as far as power requirements or the systems that you have in place. I think it is best for you to do the research so that you can make the right decision for your boat. This forum is a great place to start and return to when you have more questions while doing your research. Not only that, I have already explained that I made some bad choices that were costly that could have been avoided. I really would not want to lead you down the path I took.
