The problem is so bad that your Airstream's auto-destruct system has been armed. LEAVE NOW!!!!
Just kidding. I believe it means your fresh water TANK is empty. You only need the tank when you are camping without hookups. You should be able to fill it from your full-hookup site. No reason to unless, like me, you travel with small children in constant need of a potty while enroute to the campsite.