You will do best to purchase and install one of the 3 or 4 stage converters...Intelipower is just one name than comes to mind...Magnatek is another. Go to BestConverter.com...there, you will find plenty of converters...and help choosing one, if you need it. Call their customer service.
Which one you get depends on how much battery capacity you need....whether you are going to be camping off-grid....or always hooked up to shore power. If you are going to always be hooked up, you can probably get by with one battery. A lot of people put 2 6v Deep Cycle Golf Cart batteries in (you have to hook them up in a special way to get 12 volts out of them).
Just about all batteries have the potential of off-gasing sulfuric acid / hydrogen, and should be vented to the outside. The possible exception to that rule (might) be an AGM battery. A gel-type battery (as opposed to lead-acid) that more stable.
Hope this helps a little. I'm sure others will chime in as well.