O.K....from what I have seen and read...usually this is carpenter ants. They are black and usually about 3/8 inch long.
Now it could be other types but I know carpenters will absolutely chew up styrofoam. Had it happen to me and it can be verified in multiple articles. Carpenters ants like soft materials to nest in it seems. Does not have to be damp wood actually. They will seek out punky wood that was wet many times but styrofoam is also good places to nest.
You want to kill them...do not use an ant repellent either. That just makes them avoid that one spot.
There are several available ant baits that will kill the entire nest but avoid the cheap stuff at Walmart. Usually does not do it.
There was just a thread and the guy killed them all with a Terro product. I use another product but as long as it is good stuff it will get passed around the nest.
Sometimes( if it is carpenters) they will set up like a satellite nest in an RV while the main nest may be in a tree. Thats what I had in mine.
Check at night after dark. Go in with a flashlight and look for them. See if they are trailing out of the RV or where they are coming and going from.
Then decide on a bait and kill them. Simply spraying an ant killer won't get the queen and the nest.
Here is the website I use but there are other products as well....not just these guys.
Last..try and make sure you have an idea what ants they are..but I will bet carpenters...but try and verify.