In my state the police will come after you for having a few beers in a state park campground regardless of vehicle, and would likely try to tack on dui as well if given the opportunity. Most people can and do get away with it but one complaint or you pissed off the wrong piggy and they're gonna kick you out, ticket you, turn over your belongings, and god help you if they find anything else (weed, other drugs, anything really)