A Marine Expert???
To install a battery charger. At the risk of sounding obnoxious, this is something you should do yourself. There is no expertise required to install a battery charger. 110V going to it, which is already there with the old one, and the charger 12v going to the batteries. As to the bulb, I think you are referring to the one that indicates it is putting out 110v. It is possible this is a result of low battery voltage, but that is a guess. My Xantrex inverter has a little bar graph that shows how many amps it is drawing from the battery, and the out put voltage. Plus a little idiot light that indicates it is turned on. Again, I don't want to sound obnoxious, but you need to learn to do things like this yourself. There will be times when a "Marine Expert" is not available. Get a decent digital volt/ohm meter, and learn how to use it. Practice reading voltages and tracing circuits. The absolute worst that can happen if you will be out a few bucks for the meter. At best, you may get yourself out of a situation, where no help is available. I hope I don't offend, and certainly have not intended to do so, but if you are going to cruise on a sailboat, you need to learn the basics of electrical and mechanical repair.