Leisure battery ventilation

I have a 2008 Autosleeper Devon.  I am replacing the leisure battery under passenger seat but can not find a vent hole for the battery vent pipe to the open air under the seat box.  I have contacted Mark at Autosleeper re this but he replied that the leisure batteries were fitted by Mercedes and he does not know if such a hole exists.   Can anyone tell me if there is a hole through the cab floor  if so where please. Without one there is a serious risk of a gas build up, in the vicinity of a battery charger, plus I dont really want to drill a hole through the cab floor.