Having an open beer in the living quarters of your RV on any public - or private for that matter - parking lot isn't likely to violate any specific motor vehicle laws in most, if not all, States, but calls for research. However other violations may occur...Now if that single beer leads to a few more coupled with abnormal behavior or with that single beer you are behind the wheel and law enforcement is summoned, for whatever reason, then expect to be subject of a DUI investigation, being in charge of a motor vehicle while intoxicated or at the least a Public Intoxication investigation. If a citizen calls in and reports a 'drunk driver in an RV at Wally World ' the police are bound by duty to respond. The ensuing investigation will likely result in a pretty thorough background, criminal history and driving lic. check of RVer along with proof of insurance and registration and at the very least a cursory visual examination of the vehicle, interior and exterior. Anything discovered in plain sight or uncovered as a result of the investigation is potential future evidence against the RVer.
In reality, it isn't just a matter or whether any resulting charges are enforced by the City or County criminal prosecutor's office resulting in a court appearance etc. The police response could result in an arrest (probable cause exists on any of the scenarios mentioned) and your vehicle subject to impound. So after spending a night or two in the slammer you may be released either by posting bail/bond or in your own recognizance (no money up front). However you would still have to pay impound fees, (several hundred$$$) for the storage and subsequent release of the vehicle. Even if no charges were filed and the matter was dismissed before ever reaching court, it's one hell of a costly inconvenience. Oh and don't expect to get reimbursed for the towing and storage fees.
Now doesn't all that make you want to slug down a cold one...?
bt