The cable "Inlet" located on your trailer is a connector that looks like the connector on that wall plate shown above (the one on the bottom) it may be covered by a SQUARE cover like the one someone else pictured or by a small rubber like cover that is round. It may be on a square or round benzel or it may be in a compartment (Basement compartment) and on my rig there is a modular telephone connector sharing the standard size wall plate where it connects. (This is rare).
On motor homes such as I have I have seen it in all the following locations, ALL on the driver's side
1: Wet bay, along with hose connections and valves
2: "Converter bay" (where mine is) this is the only outside bay dedicated to electrical stuff. If you have a basement bay with cords and breakers or other electronics, Look there.
And on Many RV's. on the outside wall driver's side about 5 feet up from the ground.
To Select Cable:
On that wall plate pictured above.. TURN the switch off, (LED will go out, Antenna amplifier will no longer be powered, Park Cable will be selected)