After three days of showers an cooking and washing dishes we will have some water left in the 100 gallon tank. We might get four days but probably not. When we shower we don't use the Navy method nor shower together.
One reason I desire something like a Prevost with a 240 gallon fresh water tank, I believe.
We always make sure the tank is topped off before we leave a campground to go to the next designation. Not having enough water on board has caused us not to stop and spend a night or two boon docking, like in the Big Bend.