I've been dealing with deep cycle batteries in the marine industry for years and have seen numerous battery installations inside cabins. There is a lot of battery venting hub-bub on the Internet and most of it is "feel good" opinions. In order to present a hydrogen explosion possibility, you would need a ppm concentration that far exceeds a single or dual battery setup. A colleague, who also was a submarine sailor, once told me that it would take a room full of batteries all gassing at the same time to even come close to causing an explosion.
That being said, everyone is going to have their own opinion about it, created largely by the Internet, but it's really up to the owner how he wants his rig set up. Just do it based on some actual research, facts and what you are comfortable with and not something that has been constantly parroted online.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner