Thanks to C Schomer and westend! Good feedback, and I'll investigate the other brand of battery box. It just might fit!
I'm more familiar with Coast Guard rules than DOT, but in any event, the main purpose for ventilation is to get rid of any hydrogen gas generated during charging. Explosions due to stray sparks (even very little ones) are a possibility. The problem that I can see is that there is little in the way of definition of what adequate ventilation means. So it seems to be an "anything is better than nothing" situation. If there ARE specifics someone knows about, I would appreciate being directed to it.
The other issue, of course, is battery acid. Most of this would come from splash outs around the caps, or dripping while checking levels. My boat has carpet burns from trolling batteries, where the acid splashed out every time we hit a wave. Mfg. only installed a tray. Those are AGMs now, which resolves the problem. But, in the rear, the starting battery is still in a tray, and as the ride is much smoother back there, acid splash has never been an issue.
Presumably RV's are like cars - the ride for the battery is orders of magnitude smoother than a boat, hence splash outs are not a concern. But I don't have much experience here, hence the questions.
As far as "fumes" - I was referring to the hydrogen. There may be a bit of additional HCL (acid) that gets in the air, leading to a mildly corrosive environment in an enclosed area. If this IS something to pay attention to, I would appreciate it being noted.
But to assume there are clouds of really bad stuff that must be kept away from occupants seems to be unsubstantiated and is not the reason ventilated boxes are used. Explosions, yes - disease from "chemicals" - no.
I have also found some cheap plastic bins at Ikea that just might do, so I'm out to the garage to see what can be lashed together using those.