I have had to use the more serious method before to locate the tank drain. I got under the rig with a flashlight in the vicinity of where the water fills and searched for a hose hanging underneath.
In one case, it was recessed into the bottom of the unit and had a plastic valve in the recess that I opened to drain the tank out.
They are often located near where your water pump is, so look around there.
I like to drain, then leave it open as I drive around for awhile, uphills and down and around corners. Quite a bit more can often come out.
I have also had to repair a rig that had a cracked tank drain outlet valve because someone left too much in the tank before me.