I have almost your identical setup. 4 6V golf carts wired to a 30A charger, and a starting battery wired to the engine alternator
. A Blue Sea combiner relay makes the engine
charge the house bank and makes the charger charge the starting battery. Disconnected from both sources, the combiner relay shuts off and I can drain the house bank while the starting battery keeps a full charge.
Based on the meter on my shore charger, it rarely puts out more than 10A unless the batteries are really flat. So I'd give your 20A charger a try, it may be just fine. Once you are at the dock
, it doesn't really matter if it takes 4 or 8 hours to get a full charge, you'll probably be there for a while.