Uhm, Hydrogen gas, being lighter than air, will quickly rise to your roof and concentrate there, if it has no means of escape, like an open roof vent. It needs a spark or open flame to set it off, so if you don't smoke, it's probably more likely you'll get struck by lightning.
On the other hand, I've seen threads on these boards alleging that extreme corrosion of metal battery compartments on some motorhomes are due to ACID FUMES given off by the batteries when charging. I certainly wouldn't want to be breathing THAT ****!
Finally, let me just remind you youngsters that the batteries on the old VW bugs were actually under the rear seat, inside the passenger compartment, and we all managed to survive driving around in those!