We almost exclusively dry camp. With our first TM, 2417 Sport, we only had 8 gallons of gray capacity. I emptied about 5 gallons a day into a bucket, and that emptied the tank. If DW took a shower (every-other day), I got a full 8 gallons out.
I can take a Navy shower with less than a gallon, but she tends to use about 3 gallons per shower. Never leave the water running while washing hands or brushing teeth. If doing dishes, wipe all food chunks off plates with paper towel, to minimize water needed.
Dry camping, or camping where you can't easily empty gray tank, requires paying attention to water use. It is not nearly the same as water use at home. It could be argued that we use way too much water at home, since it is always available, and always goes away with no effort.
We can go 6 nights with 40 gallons of fresh water. With a 30-gallon gray tank, and a 6 gallon toilet cassette, that almost balances out with input/outflow. We drink water from the fresh tank, so it is almost a perfect balance, for 6 days. Beyond that, I need to empty gray, and fill fresh.
Hope this helps your calculations.
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