I think a German U-Boat might have been a little more airtight than a campervan? if it wasn't, then far more German sailors would have died from drowning than being gassed. They also had a hell of a lot more batteries and of much less safe design so it is hardly a relevant comparision for all sorts of reasons TBH.
I don't think it is a matter of manufacturers "getting it wrong" when it comes to 'suitable' i.e. plastic battery housings, it is more a matter that there is really not a real requirement for it, otherwise it would be done. I would be interested if anyone could name a single factory-product motorhome where the leisure batteries have been contained in a plastic housing.
As far as under seat installations and venting, it is actually not uncommon to fit vent-tubes to wet-cell batteries and have them fed outside under the seat and it is very easy to do as most floors under seats (VWs certainly do) have rubber bungs to accomodate wiring looms and you just push the tube through the bung. If it is an SLA battery, then it does not need venting, simple as that, as long as you have proper charging facilities.
If I fit a cheap battery (say an Alpha Xplorer Wet Cell) to a VW under the seat, it will be vented externally. If it is a SLA (inc AGM or GEL) then it will not be vented.
Adding a vent to an SLA battery does not mean it is ok to mischarge it and won't stop a problem, it might just mean you find it a touch later (and maybe too late? Remember Natural Gas has no smell. they add a chemical so a leak can be detected by the human nose. What would happen if people couldn't say "I smell gas"? (which they don't. they smell the additive)).
Finally, surprised there is no drop out for the service battery in the Iveco? In the Sevel vans which have the batterys under the passenger (RHD) footwell there is definately a hole there open to the outside world for the wet cell starter battery to vent out to.