I wish that was the case, I even tried changing the order that I placed them. Even in the stages during assembly and launch, changing anything doesn't change the drain order when I tried. I read in the change log that lower placed tanks drain first. I don't really have a clue on how I can move the spare tanks lower than the shuttle without changing the shuttle's COG. I'm not sure where I saw radially mounted RCS tanks drain first. Edit: the picture is smaller than I expected, the top RCS tank is on the external fuel tank, the other is under the shuttle.