What fuse by the batteries?
The only 1 I can think about in this location would be inverter fuse, who is >=100 amp.
Converter has 2 fuses/breakers on its panel.
1 is AC breaker who is disconnecting 120V and the other, depends on the converter size is ca 40 amp fuse on 12V feed.
Usually flipping 120V breaker disconnect the converter, but if it is shorted inside, it should burn the 40 amp fuse.
Disconnecting both fuses will put converter out of business and allow use the camper with truck charging.
Now buying battery charger those days is tricky. You can't compare $30 charger to $200 converter.
Lot of bigger chargers have automatic desulfation mode, what brings havoc to RV use. Than cheap trickle chargers will go as high as 16V, what can boil the battery dry.
I am using 20 yo converters and when I don't like the 14.2 voltage they supply for long storage, they work fine otherwise.