I think it might be easier to mount it on a window and run the small wire through the window gasket and on inside. The sharp bends needed to run it through a marker light hole, the weather sealing, the finding of the wire inside the coach and so on seem to me to be quite a project.
I do not mean to be snarky when I say this, but if you need to ask how to do this project of marker light removal and wire fishing, it may be beyond your ability to accomplish. How daring are you feeling? Grin.
If I were to do it personally, I would probably drill through the skin close to where you want the wire to wind up, then carefully seal the resulting hole. The problem there is you really don't know what is inside where you are drilling, and might hit some other wire with your drill.
It is a tough call. Window mount would be my preference I guess.