After arrriving at a state park to find the water off due to a problem with the plumbing we learned our lesson and started keeping the tank full. Did not notice any drop in the poor fuel mileage and since we are now able to stop and use our own facilities whenever we need too.
We always keep water, have had more than one occasion with the campground water going off and it was handy to have. We are also on a well at home are way out of town, when we see storms in the forecast I fill the tank, that way if there is a power outage we have a place to shower, do dishes etc.
Nothing sucks worse than a head full of shampoo and the water go off.