growing in a trailer draining the water tanks

I have been scaning the internet for the average location of the valve to drain my water tanks. I want to leave it open so the runoff can go down the drain without causing build up.

depends on your tanks

well I have a water leak at the back of my toilet.. there is not water connected to it so it must be old water coming from somewhere. really confused right now and the only thing i can think of is that water from the tank and it needs to be drained some how.