Put the following search into the shopping site of your choice:
30a twist lock to rv
There's some chaff (50A connectors, 15A connectors, etc.) but it's pretty easy to sort that out. There are multiple correct items in the return for that query on CampingWorld.com.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill