I keep 20 or so gallons for road use and potential emergencies. I don't believe in carrying extra weight if it can be helped. To answer your question, if there's a person at your destination, you can almost always bet they know where the water is.
We always travel with a full (100 gal) fresh water tank. Last year we pulled into our favorite full hookup rv park in Fort Bragg, CA and about an hour later someone hit a water line backing into their site, which shut down the entire section we were in. It was the next morning before they were able to get the problem fixed. So while everyone who showed up with no water in their tank scrambled to find a place to fill up, we were all set. I never leave home without a full tank. I don't believe the possible negative effect on fuel mileage is significant enough to worry about.
1997 Monaco Dynasty 40'
Cummins 8.3 325 HP
2008 Ford Edge Toad