I have a 40 gallon fresh water tank. We took 2 quick showers, a few toilet uses and a small dish washing and my tank was showing empty. What is the average dry camp water use? Seems to us, 40 gallons ...
And perhaps you will have lots left with only one light on the sensor. It's a bit of a pain, but filling the tank, then having someone watch the sensor lights while you fill a bucket from the outside shower will really tell you how much is left. Get a couple of measured buckets, and empty the tank until it runs dry.