My new WIP takes place inside a well-known theme park. I know it is OK to use locations in books within reason, but the entire book takes place inside this theme park, and terrible things happen, albeit nothing slanderous/libelous/offensive to the park specifically.
Can I do this? Or do I have to make up a pretend theme park?
You mean like Murder on the Orient Express?
Or Murder at the White House?
My guess though is you mean Disneyland or Disneyworld. I can tell you the Mouse House is fierce about brand protection. If you want to see what I mean type "Murder at Disneyland" into the Amazon search engine. Not much pops up.
Which says to me that what you'd think would be a target rich environment for murderous writers (I mean, really, don't you want to kill someone after hearing It's a Small World After All for the n+1th time?) has almost no hits. Which leads me to think that publishers put the kibosh on the title, or the legal wolverines at Disney do. It doesn't matter who pulls the plug.
On the other hand, Robert Crais (the recently named 2014 Grandmaster of the Mystery Writers of America) set a shootout at Disneyland and no one said a word, so there's that.
The bottom line is you can write what you want and see what happens. Anyone can sue to stop you and the legal department at a publisher can say "it might lead to lawsuits so change it." There's no hard and fast answer.
If you write it, will you be willing to change it? That's the real question.