Actually, the question is a little confusing. I am assuming you'd like suggestions on how to keep the batteries charged while the boat is in a slip with access to shore power. You used a trickle charger in the off season to maintain/protect the batteries while they were out of the boat. Also it appears that there is no charger of any kind (other than the engine
alternator) currently on the boat. You could plug
in the trickle charger on board, connect it to the batteries and keep the batteries topped up, however a trickle charger is really slow particularly if you have multiple batteries and will not equalize (condition) the batteries at all. You would be better off installing a smart, three stage charger that will recharge, condition and maintain the batteries automatically while you are on shore power. These don't have to be too expensive. The capacity of the charger depends on the number of battery banks and number and capacity of the batteries in each bank. It sounds like you really don't need anything more elaborate like controllers, monitors, etc.