If you don't see a fuse in the battery lead to the junction box, then odds are when the wires are connected to the battery posts your trailer is powered. Cut off switch is easy to install. Popular one is this:
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.