If you vent the batteries be sure it is a box that is vented not just a vent for the larger area where the batteries are sitting. All those fumes are very corrosive and will attack anything metal.
Also if you boil out the fluids you need to address where the run-off is collected or bypassed to.
The battery fumes are also very explosive and should not be around anything that can spark on you.
I personally would not like having wet cell batteries anywhere close to where I would be sleeping. To me batteries belong outside on the trailer tongue but you gotta do what you gotta do sometimes...
If money was no object here I might think about getting some sealed AGM type batteries if they were to be used inside the trailer.
Roy Ken