There are useful notes and examples on accessing parts of JSON-structured KM variables at:
token:JSONValue [Keyboard Maestro Wiki]
perhaps worth adding that if you want to create such variables using a Set Variable ... to text action, any back-slashes in newlines (\n) or tab (\t) characters within quoted strings need to be doubled.
So, for example:
to create a KM variable value like:
{"a":9, "b":"bravo", "c":null, "d":"a\nb\nc\nd\ne"}