In Maryland and Virginia the law says "NO Alcohol" , but personal experience shows the law is not enforced unless necessary.  If they see you with an open container they'll say some thing. If your a rowdy drunk they'll do some thing.  Keep it cool and they don't care.

NC=no alcohol in SP's.  I believe SC was similar.  Of the SP's I've seen here in Eastern WA there's been nobody around to enforce anything.  Even payment's on the honor system.
My 2centavos, keep it in a coffee mug or your pup.

I talked to a ranger at Raccoon Creek SP here in Pa. He told me they don't enforce the law. They know people have beer. The rule is there for the times when people get stupid.  So just don't be stupid. Was what he told me.  OF course other parks may operate different.

Its the same for New York no alcohol allowed. But enforcement is controlled by your own behavior.

Most of the postings here on the subject are quite true.  Discretion is the key.  Should there be a "problem" and there has been alcohol involved, the camping park folks (and the sheriff) have the upper hand.

Yeppers - the rangers in the SP's in NC will speak with you if you have a can of beer, but, they won't bother you with a red solo cup as long as you are not rowdy/loud/obnoxious. 

We've done bloody mary's for breakfast for many years and never had an issue "red-cupped", and evening pre-dinner drinks weren't a problem.

We just don't advertise our beer/liquor and no problems.

Back in the late 90s, I camped several times in Cheesequake SP in NJ.  One of the rangers was called Barney by the campers.

It was said that Barney would hide in the bushes to catch campers drinking.

And then there was the bathroom nazi who would clean the bathrooms and then sit in the truck and keep campers out until the appointed hour.

Joel