Silver350 wrote, "...I have seen diagrams on this site where the vent tube went down through the floor. Where is the hydrogen gas going? I think its collecting in the box..."
Here's a diagram I posted with the 2010 SMB West installation when done inside the van for two AGM 27s (4D batteries are usually mounted outside and beneath the van):
Other than the openings SMB put in them, the cases are completely sealed - no handles. The Dekka literature for AGM battery installation recommends adequate vehicle compartment ventilation. SMB put drain holes in too so that any water that comes in via the vent cover on the side of the van can escape - and, yes, in case a battery leaks.