When I initially took delivery of the toy hauler last May, I requested NO battery installed. There was a 12 volt battery prep, which meant a vented box "just" large enough of ra typical dual RV/Marine battery.
I had already decided I wanted very large amp hours, even though I typically came home from a 3-4 day trip with well over 80% sate of charge with 2 12 volt batteries on my previous small camper. I know running the furnace heavily is the big power draw when the weather is cold at night.
So on this camper, I removed the vented box, and installed a dual 6 volt box, and cannibalized the old vented box for the vent top and bottom. I installed these onto the new box. I had to get the bottom hole perfect with the hole saw so everything lined up with the floor opening (lower vent hole). I also laid some of this felt that has an anti-acid solution impregnated on it so if there are any small battery spills they'll be neutralized in the box.
I can say from experience, a 62 pound battery is a beotch to lift into a plastic box that's at chest height!! And I had to do it twice. And I had to slide the plastic box with BOTH batteries already inside it over to the mounting location without damaging the lower vent pipe. I am not looking forward to taking them out for the winter. I may install some kind of table slides under the box.