Because changing lanes in heavy traffic is difficult, at best, and other drivers, when alongside the coach, can't see the rear signal lights and in bright light the arrow in the mirror in almost invisible (and drivers normally don't even look up that high) they don't know you are trying to move over. Installing a "midship" signal light at least gives other drivers a clue as to what your intentions are.
Purchase an amber colored marker light that has a 2 filament bulb and a base slightly larger than the marker light you are replacing because the existing light is set in a recess on the wall. The light should protrude out far enough from the side of the coach so that you can see it from behind the coach. These lights are easily found in truck supply shops.
Run a single wire from each light back to the rear signal light (or the front light whichever is the closest) on the same side, and splice it into the signal light wire. Inside the bay, wrap the new single wire and the existing marker light wire with tape so that they become "one wire" and move together when you open the bay door.
Remove the existing marker light. Attach the new wire, the old marker light wire and the ground wire to the new light and install it on the side of the coach using the provided screws and clear silicone caulk.
That's all there is to it.
Click any image to enlarge.