Gassing of battery

Bought brand new motorhome. Within day had red flashing light on display re batteries.Took it back and they replaced a fuse. Hooked up and van was full of sulphur fumes and battery no good. New battery now and seems Ok but I would like peoples opinions as to the dangers of the gas produced and what could have happened if we had been sleeping inside. The habitation battery is under the bed!

I spoke to a couple of battery specialists and they stressed how dangerous this was but the sales garage tried to dismiss my worries.

The brand new vehicle seems to have been manufactured three years ago. Do you think the battery failure was due to the vehicle standing around in storage etc?