It would be great if you could move the propane tank up close to the tongue jack. This would give a wide spot on the tongue to mount a metal box even large enough to eventually mount four each GC2 6V golf Cart batteries...
Another good point here is the front wall wall of the trailer is a pass thru storage area where you can mount INVERTER, FUSE BLOCKs, 12VDC SUB PANEL, etc to support all kinds of camping off the power grid ideas that will be within just afew feet of the battery bank.
Consider this typical smaller trailer setup for running your home entertainment center and some other 120VAC items from two drops from an INVERTER that is direct connected to the battery/
A typical 48-inch metal battery box install for this size trailer might look something like this
food for thought...
Roy Ken