On mine (2005 38') it is inside the inner basement behind the sliding door on the drivers side under the tangle of hoses. You have to have long arms to reach it and it may be buried under the insulation. Get a flashlight and look for a hose going to where the drain goes through the bottom of the trailer. You can find the outside end of the drain by looking under the rig then try to spot that particular hose above the corrugated basement floor.
On the 2005 RE we had, the tank drain was located in the basement as motor31 said, except it was on the passenger side/trailer door side. Yes, it was down deep and hard to get to.