Ron and Nelly, I can tell you from experience of the bad kind it's not a real good idea to leave the dump valve closed in your situation. Our 2003 3295RK was plumbed like yours. Our particular washer could put about 2 1/2 loads into the gray tank before it backed up into and overflowed the shower, flooding the upstairs floor, running down into the basement at the various drain lines and other openings, thoroughly drenching everything in the basement including the carpet. And it excites the neighbors who run over and shut off your water, thinking you have a leak. Silly people. This happened to us not once but twice. No, I didn't intentionally leave the valve closed. I just got out of my routine and forgot. Twice.
How many loads you can get into the gray tank depends on how much water your washer uses and what water level you have set on the washer controls. Just be very, very careful.
If you add an additional valve at the dump cap, close that one and open both gray tank valves you could probably do four or five loads before overflowing, depending again on the washer and its settings.