Since many trailers are using various charging sources, and multiple batteries in a variety of configurations it becomes necessary to add appropriately valued DC Fuses, or DC Breakers at Critical Points.

Rule of thumb is to fuse both ends of the same wire if they connect a  Charger to Battery, Charger to Charger,  or Battery to Battery.  Especially important on longer runs.  

Fuse all load wires as close to the voltage source as possible.

Fusing/DC Breakers must be properly valued for the load and wire gauge being protected to avoid a fire.

The typical 100 Ah (Amp Hour), 12VDC battery, has the potential to weld metal, or create a fire with ease, if details are not paid attention to.

Charging sources can be, but not limited to a Converter-Charge, Solar Controller-Charger, Tow Vehicle Charging system, etc.

Below is a simplified configuration.
Integrating both 12VDC & 110VAC