If the inlet for the cabin is a single 15A plug (a normal household plug), 12/2 would be plenty for a cord unless it's quite long. If it's a 30A plug, 10/2 would be OK for moderate distances. You don't need a three conductor cable unless for some reason you have a 240V split phase inlet, which seems highly doubtful for a single small air conditioner, and is at any rate not the output of any generator that would be in a small motorhome. (The ground conductor is not counted in the wire count of cables).
Actually, with SO cable that extension cords are made from,the conductor count does include the ground. For 120 vac, you need 3 conductor (black, white and green). 12 gauge would be fine, but 10 gauge wouldn't hurt.
