Surely you add a couple of Batteries to your tongue area. Looks like plenty of room behind the propane tanks shown here.
Perhaps you can add some angle sections and extend out on both sides of the top of the tongue to fit additional batteries.
Mounting the batteries inside the enclosed area require venting for fumes and capture of fluids if they boil out on you... Not really a good idea for wet cell type batteries.
The battery fumes are very corrosive and will eat your exposed metal item very quickly..
Your best purchase is two each 6VDC golf cart batteries mounted in series to give you the 12VDC you need. Alot of folks like to mount two of these 6VDC batteries in a metal box on the trailer tongue for safe keeping. Having around 220AHs battery capacity is really nice when camping off the power grid...
Roy Ken