Grant knows his stuff when it comes to Ford chassis info. Here's some info to add to what he's already given you.
Here's a link
to the 2015 Ford Body Builders Layout Book pages for the F53 motorhome chassis. The fuse and relay information is on the last two pages. The "TT PARK LAMPS" relay is number 8 in the Auxiliary Relay Box.
1) You should hear it click when someone inside the coach turns the parking lights on and off
2) The drawing shows numbers on the relay socket. You can check voltages with a voltmeter:
14 - Input signal from parking light switch (activates the relay)
57 - Ground
1105 - 12V from fuse F15 ("hot at all times" if fuse F15 is intact and in place)
962 - Output to rear of coach
If the relay and its socket check out fine, then the problem is between the Auxiliary Relay Box and the pin on the trailer connector. The most probable cause is in the trailer connector itself (usually a loose or corroded connection). On rare occasions people have found something broken between the Box and connector.