You can temporarily roof mount your portable by using a tray that itself is held to your cargo rails if you have them, and the A/C shroud's metal base. Bungee cords work for that.
In the photo here, that's the way it stays driving down the road at highway speeds, no problems at all. Wires go down the fridge vent same as for a nailed down type installation, except to have disconnects at the roof for when the panel is elsewhere (like in a stand on the ground)
You can also use ski-racks like for your car that go to the rain gutters each side, except you have to use two to reach across the wider RV.
With a truck camper, you can put the tray up front on the roof and pass ratchet straps around the whole front end of the camper to hold the tray in place on the roof.
