I've seen motorcycle batteries with vent tube nipples but never a car or
deep cycle battery.
That Jim's trailer's battery compartment is not ventilated except for the
tube suggests that it originally had a battery that had a nipple for the
tube, as such a small hole is nowhere near sufficient ventilation for the
compartment. I would suggest putting vent louvers or a grille in the
compartment's exterior door.
I have seen batteries that had exploded. Battery acid goes everywhere.
Plastic boxes are readily available for deep cycle batteries to be
enclosed in. If the battery ever leaks the box will catch the acid. I've
seen them at Walmart, auto parts stores, marine supply, etc.
As to putting the batteries outside the trailer body, all kinds of
equipment has its battery inside the outer skin of the vehicle. As long
as the compartment is well ventilated and drained to the outside, and is
sealed off from the rest of the space inside the skin, there's nothing
wrong with batteries inside the skin. Helps keep them from sprouting
legs. If people can't see your batteries, they're much less likely to
make off with them.
My trailer is the only thing I have had that has its batteries outside.
They are in a (fiberglass?) box mounted on a frame above the A-frame.
Automobiles, trucks, motorcycles, golf car, boat, you name it. They all
had their batteries inside.
Al Grayson