Your correct on needing another ac/dc charger kind of
the battle born where drop in compatible with most ac/dc charges , as far as i understand , you can email/call them and you can confirm this , but that’s what they advertise
the go power 30amp transfer switch can be ordered prewired so there a plug on the relay box that you plug in the ac/dc charger , also known as a deck charger , so any time the rv is on shore power the charger works but not on inverter power which Is what you want to avoid
a cheap simple way I did it is to separated the 120 ac power wire in my panel for the panel battery inverter charger part , it was wired to a breaker with my a/c unit , it varies by rv , I moved the a/c wire to another breaker and left the charger on its own breaker ,
so when I am on shore power I leave the breaker on , when on inverter power I flip the breaker off , cheap and easy , as long as you remember to flip the breaker when needed
the manual for the go power which I liked better explains about disconnecting the built in battery charger on the panel ,
As a alternative which would make things easy for would be to wire the charger built into the panel to only be active on shore power via wiring from the relay box - the manual shows you how to do this , you will need to do some extra wiring and install a fuse/break
as far as I know please confirm with battle born your rv built in battery charger is fully compatible with the battery , so your only cost is the relay transfer switch and wires from the inverter to relay ( instructions will tell you what you need)
hope this helps ,as Usual Safety first