You may do the following to get the figures:
Fill each tank with a bucket and multiply the gallons of the bucket with the times you emptied the bucket into each tank. For the fresh water tank a large funnel is required. For fresh water tank it is easy, for the black water tank the toilet is the best inlet to be used. For the grey water tank(s) it should be the sink, vanity and shower. By that method you can find out what empties into which grey water tank. Very helpful for boon docking without sewer hock up. And be careful when you empty the black water tank, some residue from previous usage may flow off as well. You do not want this in your drive way or other inappropriate places. -
Just my 2c.