the 'petcock' values drain the water LINES only.
they are the low water line drain valves
used to drain OUTSIDE lines b4 winter.
to drain the fresh TANK, use the nylon drain valve ON the tank.
there is a pic of the valve in post #14 here...
this is typically on the street side between or near the WHEELs.
it' helps to TILT the trailer slightly to that side (raise the curb side)...
simply done using the road crown/grade or a plank.
consider FILLING and draining 2-3 times completely.
it may take a HOUR+ to drain a full tank ONE TIME via gravity.
the fill/dilute/drain repeat cycle can be quickened
by using the freshwater PUMP and gray tank exit...
shut OFF the pump as the tank gets low...
to avoid over heating the pump.