Looks like plenty of room on the tongue of your trailer to mount some batteries.
This is what alot of folks have done with their similar trailer setup. This 48-inch tool box will hold four each GC2 6VDC batteries is a lock box.
This is a couple of photos inside the 48-inch tool box showing vents and mounting of the GC2 6VDC battery
You can also just place some angle sections to fit your battery boxes and extend out over the top of your trailer tongue area.
Batteries need to be installed outside if possible so they can vent their poisonous fumes and drain directly to the ground if you boil out the battery fluids.
The downside however is you should make it as hard as you can to keep thieves from stealing your batteries. Batteries don't come cheap... If you make it hard to steal then the thief may go on to your neighbors easy picking batteries..
just some of my thoughts