Fishing the wires across the roof cavity, would be tough enough , but trying to get them down an outside wall with the ridged insulation could end up making a mess of your clean, new to you RV.
You would have to try and stick to installing in an interior wall and even then things could get messy.
If you are going to try , do the bathroom first, shorter distance to fish the wires across. Take down the vent trim and look in the roof cavity to see how much space there is; and how the wires are run. Drop the light out of the ceiling .Select the spot where you want the switch, then you will have to loosen the panel from the wall far enough to get to the top of the wall with a drill . Drill a hole big enough for 2 wires. Then you will need " fish tape " a stiff but still flexible wire from the hardware store, to feed up through the hole in the wall toward the wiring, if your hole came up between different trusses than the wiring this could take some doing.
Cut the power wire at the light, run two wires to the switch by attaching to the fish tape and pulling the wires back through. Attach the wiring to the switch mount the switch and wire to the light and assemble, the light and wall. Not a quick job or an easy one.