Actually you should be able to remove the rear tail lights on the RV to gain access to the wiring then run the wires down behind the exterior wall. You will then need to remove the belly enclosure material at the rear to access the wires and run the pig tail out towards your hitch for use. Now if you are wanting to add wiring that would include trailer brakes it will be a different story.
Not to mention if you need the use of trailer brakes on a trailer you are towing off of the RV receiver hitch your probably over loading the receiver hitch and frame of the RV.
__________________ 2013 Ford F350 FX4 4x4 PSD 2013 Wildcat 353LS