I always keep water in the fresh tank, even when using a FHU site. There's nothing worse than using the toilet then discovering that the campground shut off the water because there's a leak somewhere and it'll be 4-6 hours until it's back on.
One piece of equipment I have is an electronic water hardness tester. When the water is above tap water hardness levels, I don't refill the fresh tank but wait until I get to a campground with better water. There's a a few campgrounds I regularly stay at that have very hard well water. If it's just an overnight stay there, I don't hook up the water at all and just use what's in the tank. Hard water is fine for flushing out the black tank when dumping but can be an issue for a Suburban hot water heater, causing rapid disintegration of the anode rod.