Also be awear that these gasses are flamable and a spark in any enclosed area where battery is will explode.
The key word is enclosed but I always would err on the side of safety.
Gerry's right. The key word is 'enclosed'. A little bit of hydrogen in a battery box in the outside world is not going to cause any explosions. At best, a barely audible 'woof' even in the unlikely event that enough of it was able to pool up inside the lid (hydrogen is lighter than air). Inside the trailer, another story. Sadly, I think few of our trailers are sealed well enough to allow any significant quantity of hydrogen to build up. Better safe than sorry though.
More important than hydrogen though, is to make sure that wherever you put the battery, someone isn't likely to put something large and metal on top of it. Like that big crescent wrench you keep in your cutlery drawer for disconnecting the propane
tank. Or under your big metal hot water heater. Or in the cabinet with the cookie sheets. Or in the cupboard with the aluminum baking tins. See my other thread about 'welding with car batteries'.
I'm also a fan of Optima batteries but for a trailer, I wouldn't waste money on the red 'starter' batteries. You only get a few 'full drain' cycles on them before it's time to pull out your wallet again. On my 4x4's, I have had the yellow deep cycle Optimas for about 7 years and they've been drained fully as much as a dozen times and they always bounce right back. Optimas are sealed, use a gel, and weigh about half as much as your typical group 24 battery with just as much 'punch' as your typical car battery. The red ones are cheap ($110 at Wallymart) but the yellow/blue are quite expensive (I paid $380 each for mine, 7 years ago).