I made mine with three wire. The neutral and the ground in are basically the same, do not under size the wire and connect the green or white wire to the silver screw and the green screw. Now I know someone is going to slam me for this but before you do get an expert to say it. I am a Journeyman electrician.
How can you say that the neutral and ground are basically the same. This would be the same as wiring a house only using two wires with out the ground and using the neutral as the ground.
The neutral is a current carrying conductor. If you have current flowing on the grounding (bare) conductor, then anything attached to that (for instance a washer) will also become current carrying. If you were to then touch that washer while standing on a grounded surface, you now become part of the conductor and will receive a shock, perhaps a fatal one.
Plus how many RV have been damaged by electricians that have wired the 30amp 120 volt outlet 30amp 220volt?