Micro Pulse Systems Monitor Calbration

I have a 2014 FC 23D and would like to calibrate the systems monitor. The manual says to get a reading when empty then get a ready when the tanks are full No problem with the fresh water tank and getting the empty reading on the black and gray tanks. How do I fill the back and gray tanks to get the full reading?



I am a newbie and this forum has been very helpful



Thanks for your help