I assume you are talking about draining the fresh water tank.
On my 2014 GWV Legend, there is a short piece of tubing coming from the bottom of the tank with a cap at the end. It is accessible from the side of the van. You unscrew the cap to drain. I have no idea if the older vans worked that way.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)