All the fuses and relays for my rear trailer plug wiring harness is located in the same compartment as my inverter, passenger side rear. The fuses are under a plastic fuse box cover. You can locate it by turning on your hazard lights and listen for the sound of the relay chattering.
"If you think it could rattle, it will"
2006 National Tropical LX T350, CAT C7 350HP, MH3000
2014 F150, SMI Air Force One, Blue Ox Avail