Yes, they do need some ventilation. The attached article gives the rationale behind this. It's mostly a precaution, but could be pretty explosive if the right set of conditions were encountered.
AGM Battery Venting
2016 Ford F350 Turbo Diesel SRW 4x4
2017 Grand Design Reflection 297RSTS
2013 Ford F350 Turbo Diesel SRW 4x4 (Destroyed by fire - 8/29/16)
2014 Grand Design Reflection 337RLS (Destroyed by fire - 8/29/16)