Q. What is it supposed to be on the show?

A. A KOA Campground near World's End State Park.

 
Q. Where is it supposed to be on the show?

A.  In Pennsylvania.  

 
Q. When did we see it on the show?

A. In Episode 1 of Season 2
 
Planning to kill Al (the serial killer known as Rapunzel), Dexter tracks him down to a camping site in Pennsylvania, where Al and his family are vacationing, while living in a giant RV.
 
Dexter shows up unexpectedly, pretending he just wants to be friends, but Al is definitely not happy to see him. He likes to keep his two worlds separate and doesn't like Dexter showing up around his family.
 
Dexter tells him that he is worried after what happened with Prater, and when he heard about Al's most recent murder, he just wanted to get back in touch with one of the former members of that macabre club. 
 
That seems to mollify Al a little, and he agrees to meet Dexter at a former body shop nearby.  But both are planning to eliminate  the other.

Q. What is it actually in real life?

A. A state park, but not in Pennsylvania. 

 
Q. Where can I find it in real life?

A. There actually is a World's End State Park, in Forksville, Pennsylvania.  But this isn't it.

They filmed this scene at Tallman Mountain State Park, which is near the coast of upstate New York, just west of  the little town of Sparkill, NY. This large park is a little out of the way for Dexter, but it's still in New York state.
 
Tallman Mountain State Park is a 687-acre state park in Rockland County, New York, located near the  Hudson River. The official address is Route 9W, Sparkill, NY 10976. 
 
You can find there website here.

The sign reading “KOA” stands for “Kampgrounds of America”, a real-life chain of campgrounds throughout the country. But as far as I can tell, there is no KOA campground at either Tallman Mountain State Park or at World's End State Park, although they do have multiple campgrounds in both New York and Pennsylvania. ) 

If that town name Sparkill sounds familiar, that's because it's the same hamlet where they shot the Iron Lake scenes in season one of Resurrection.  That included the early scene of Dexter escaping from the hospital there, and the scene of him stopping at a store to buy new clothes. They also shot the scene of Angel Batista talking to the deputy in Iron Lake, and even the scene of Charley killing the Canton Clubber. 


Here is a matching StreetView of the park entrance: 
 

 
Here is an aerial photo of the location.  And here is a map link.