A topic discussed many times. I may have a solution that doesn't require adding a cut out switch at the battery. This should provide full cut out at the factory battery cut out switch.
The main 12 volt cable from the battery goes through a manual reset breaker, sized based on the trailer and it's loads. It then feeds 2 buss bars. One is on the switched side and one stays powered at all times. If you can locate these buss bars, a wiring diagram should be displayed near by. My buss bars and diagram were located in the front compartment, above the battery box. I had to enter the left side of the compartment and look above the battery box.
The one item that must remain powered at all times is the safety break away switch. I plan to remove that wire from the un-switched bus and install a Bussmann CBC-20HB Circuit Breaker (Type I Heavy Duty Automotive with Stud Terminals and Bracket - 20 A)
to feed the breakaway switch.
I will then disconnect the bus feed from the un-switched side of the cut out switch and move it to the switched side so that all 12 volt lines except the break away line will be disconnected when the cut out switch is in the off position.
I plan to display a modified schematic near the buss bars for anyone doing future checks in this area.
Any advice or cautions are welcome.