Hitch locks are part of the puzzle. It is virtually impossible to deter a determined thief, but you can make it so inconvenient he moves on to an easier target. You also dissuade the thieves of opportunity that are looking for a quick score.
A hitch coupler lock
is one item. Many brands, many types, same concept - deny use of the hitch.
Second, some sort of wheel/tire immobilizer. Again many brands, many types, and many prices. I went with a cheap one
The Airstream door seems sturdy, but there are other options for internal security which I may explore in the future.