I cannot disagree that the OP should have been using a 50A RV receptacle.
We can sit around all day and act like keyboard commandos with comments about electrical codes and how many children are going to die because one little 120V circuit (that's probably de-rated anyway due to voltage drop) is installed improperly...but what's the point!?!
A non-weather proof adapter was sitting in a pile of dry leaves and exposed to rain. THAT'S WHAT HAPPENED!
The NEC is a minimalist guideline and is superseded by the state code, unless the state defers to the NEC. In all this "not installed per code" talk, not one single "expert" has yet to show how it fails to meet code. I would further venture to suggest that not one person on this forum has an RV that's wired any better than that! Nearly every house in the country will fail to meet code if the inspector looks close enough.