Follow the plumbing that comes out of the bottom of the tank. Somewhere between the tank and the pump (usually only a very short distance) you will find the drain valve. If you open it and nothing comes out, then maybe you can check from underneath and maybe a mud wasp or other insect built a nest in the drain tube....
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter