VAN CAMPER COVERSION

If the channels are on the interior of the vehicle, not exposed to the elements, Romex (NM-H) in flex tubing or otherwise protected against abrasion and cutting and properly secured against flexing/vibration is fine and typical. If exposed outside, I'd probably prefer EMT with wires suitable for wet locations or liquid tight tubing or similar. Generally RV 120V wiring is not run where it is exposed to the elements if it can at all be avoided (e.g. for everything other than connecting to a built-in generator).