You might not be able to tell by insulation color. The ground would be identified as the wire running to a bolt, screw, or the metal frame of the RV. the other wire would be the (+) wire and could be tested by a multimeter or test light to a ground.
The headlights often have short ground wires from the back of the lights to a nearby ground. The switch probably also has a ground to complete the circuit for any illumination bulbs.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI