The battery is provided by the dealer - does not come with a battery from the manufacturer...
The wet batteries are most always in the very front compartment under the bedroom overhang area where venting is easy to accomplish. If it was originally delivered with ATM sealed batteries they could be almost anywhere. Some were installed under the pull-up bed platform...
Like mentioned above the US DOT folks require you to have a working battery setup to operate the emergency Brake operation in case of a trailer disconnect from the tow vehicle... This is for trailers that have Electric brakes installed... This is an enforceable state law if the trailer is on US Public roads. Be sure to have all of this checked out before pulling your new to you trailer home over public roadways... If you are buying this from a legit dealer they probably will want to install a battery as it may be a liability to them.
Being a 2016 model sounds unlikely to me it doesn't have an installed battery...
Roy Ken