Gray tank capacity

Our camper has a 30 gallon gray tank for bath and kitchen. I have a blue tote with 22 gallon capacity. When we are dry camping I dump every day. Two showers and normal daily activities, washing dishes, brushing teeth, etc. usually puts about 15 to 20 gallons in the tank. On the next to last day I don't dump. That will give me 4 showers and two days worth of normal use and the tank will be full when we get to the dump station on the way out. A long way of saying, yeah 4 showers can use up close to 30 gallons.