I am dumbfounded to finally realize there is a reason why there are only 3 valve pulls on our recently acquired 2013 415RW, there are only three tanks. This is astonishing only because in no area of information that I have come across suggests anything less than 4 tanks; one black and 2 GREY up front and one black/grey combination for the rear half bath. Are 3 tanks a normal configuration with 4 being an option? Apparently the galley drains into the rear grey/black as opposed to having its own grey tank. Also, the function panel has a #2 grey tank level indicator. Does it read the level of the rear tank or is it just a "dummy"? My intention is to retrofit that forward #2 grey. Thank you.