A far as I'm aware sealed batteries are, to all intents and purposes, just that. In a normal charging situation neither AGM nor gel batteries will vent hydrogen. It seems they do have a pop-off valve built in to the casing which will release any hydrogen given off if the battery is seriously overcharged, but only then.
I guess whether or not you feel you should vent either of these types is down to how likely you think it is that your battery may be inadvertently overcharged - i.e. by too high a charging voltage. Any hydrogen given off is lighter than air so will rise in any case, so an open drop-out vent (one in the floor adjacent to the battery, for example) won't facilitate hydrogen exiting the van per se. It will permit more movement of air around the battery, but other than inside the fridge there aren't really very many places in an Eriba which aren't reasonably well-ventilated anyway.
If the battery you end up getting has a vent spigot built into the case by all means connect a pipe to it and drop that through the floor, but unless the battery is accidentally and badly overcharged it won't do anything.
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