I'm also guilty of dumping my grey water in the dunes. I have 70 gallons of grey water (two 35 gallon tanks) in my rig, one in front and one in back. I try to dump mine in the cover of darkness or early in the morning. Dig a deep enough hole, 2 to 3 feet deep, crack the tanks, and let them slowly drip for about an hour. Once there empty, fill in the hole and you'll never know I dumped the grey water.