Good luck finding an on-board charger rated less than 5 amps per bank. And make sure to get another set of batteries for fishing back to back days. No way a 2 amp or trickle charger will top off deep cells overnight.
Yeah, I think he is mistaking an onboard charger for something else because the onboard chargers are designed to step down amps once they reach a certain percentage of charge. They are the best solution to charging deep cycles. No way you can trickle charge a deep cycle battery overnight with something under 2 amps.