In order of MY preference, but I used option 2. (I carry option 4 in the car trunk as emergency backup.)
Option 1:
drill a hole in each tail light fixture and install a new 2 filament bulb. run wires to the RV's 7 pin connector
Option 2:
put diodes in wire going to car tail lights and run wires to the RV's 7 pin connector.
Option 3:
make a tail light bar that installs in the car's trailer hitch. run wires to the RV's 7 pin connector
Option 4:
put magnetic tail lights on the back of the car and run wires to the RV's 7 pin connector.
That is all I've got.