If the main panel is in the cabin, you would have to run 3 wires plus ground from that panel back to feed the RV outlet on the pole in addition to the feed from the pole to the cabin.
If you put a small outdoor main panel on the pole, that could feed the outlet and also feed the subpanel in the cabin. You would need a good earth ground system at the pole, and you would run 4 wires including ground to the subpanel.