You have the right idea about the land surrounding the RV park and Old City - think scrub desert giving way to a wasteland.  Old City might resemble a larger version of Las Vegas, but looking like a gigantic ghost town.  Of course, everyone knows it isn't COMPLETELY empty ... imagine horrors like midnight ghost wolves, Moorlocks, and such ...  If you recall the twin Wraiths from Matrix, there might be creatures like that, the stuff of nightmares.  So if you don't have to go into Old City, best to keep out.  Of course, try to tell the heroes that when their family needs supplies ...

Carnivale was a TV show heavily steeped in magic and prophecy.  It was essentially a war between good and evil.  I am just referring to the Barnum and Bailey type freakshow stuff, maybe even a fortune teller.

Ultimately I want the members to write the story.