If the connector behind the bumper checks out, the next suspect area to look is where the wiring harness traverses the frame around the back edge of the front wheel well. You need to pull the front wheel well liner.
For reference here's a list of all the trailer tow plug fuses:
F53 ATR 20 amp Left Trailer lights
F81 ATR 20 amp Right Trailer lights
F29 FMM 20 amp Trailer Park lights
F28 FMM 20 amp Trailer Back up lights
F30 FMM 30 amp Direct from battery
F11 J Case 30 amp Brake Lever
F87 ATR 10 amp ITBM module