What size sub panel do I need
To answer your question, the smallest sub panel you can find. You would need to use conduit and correct fittings for running wire. However, I wouldn't buy a sub panel for just one double poll breaker. Instead, since this is a sub panel we're talking about here, I would set up a two or three breaker sub panel. 1 or 2 for the lights and other outlets, and 1 for the RV outlet. For the RV outlet, if it is a 120V outlet, which I think it is, they make up to 50 amp single poll Square-D breakers. Although, you would need a 40 or 30 amp single poll. For the other breaker(s), I would use a 20 amp. Just a suggestion...