It's not exactly easy, but you could pull one of the two 30A fuses in the converter. On my Elixer converter if one of these is blown, it will stop the battery from being charged.
Alternatively, you could disconnect the converter charging wire from one of the two 30A circuit breakers in the battery box. That's probably easier than pulling a fuse. Here's how mine is wired, your's could be different: https://www.hilotrailerforum.com/att...ary-wiring.jpg
It's the #8 black wire on the extreme right side in the diagram.
You could also put a simple ON/OFF switch in that wire too, up next to the circuit breaker, to simplify cutting it off.
Beyond that, I don't know of any way to disable the converter battery charger when the converter is getting power from a generator. Disconnecting the battery would also stop charging, but it would kill all your DC equipment too.