If your rig is like the vast majority of RVs you have ONE black water holding tank which collects waste from only your toilet(s) and ONE gray water holding tank which collects waste from the kitchen sink, the bathroom sink and shower. If you do dishes 2 or 3 times a day, take leisurely (non-navy) showers, you will fill the gray holding tank MUCH FASTER than the black tank. If you have a washer/dryer, the gray tank will fill 5 times faster than that which is why you must keep open the gray valve when doing laundry. When your monitor indicates one of the tanks is getting full, dump them both.