[rheavn]

I'll tell you where mine are:
R-1 compartment-I drain this tank from the door of the compartment
Front of RR wheelwell-I drain this tank standing outside the wheelwell.

Front: I drain all of these air tanks from the RF wheelwell.
Two large oval shaped tanks-front brakes
One rectangular crossmember that runs just behind my generator housing.  Visible from RF wheelwell.  Services RF air spring.
One rectangular crossmember that is above the two front brake tanks.  Services the LF air spring.

Rear: I drain all of these tanks from under the rear of coach.
One rectangular crossmember that runs above my hydraulic cooler.  Services both LR air springs.
One rectangular crossmember that is just behind the rear end housing.  Services both RR air springs.
A large oval shaped tank above & between tag axle.  Services rear brakes
One rectangular crossmember that runs above the transmission behind where the driveshaft connects.  This one is two air tanks in one crossmember and need a drain on each side.  Services the left and right tag air springs separately.

All crossmember air tanks run from the left side to the right side.

NOTE: Not all of my air tanks had drain valves when I bought it. I removed the plug and installed a drain valve.