If you have wet batteries, the water you replace has turned into H2 and O2. H2 rises, and will burn / explode on wide mixture ratios. Sealed batteries probably make minimal amounts in normal use (but then things do sometimes fault, and sometimes overcharge). There's disagreement on how much of a problem it is ( https://www.caravanersforum.com/viewtopic.php?t=7574#p100660 ).
I'd guestimate the easiest way would be to just run a wee tube out the top of the box, up and out somewhere. Could probably hook it over at the top like a candy-cane to easily keep water out.