I always fill before a trip. It does NOT affect my MPG at all (get the same mileage when I drive home with the empty tanks).
As I stay at public campgrounds (USFS, COE, BOR, etc.), filling up at the campground is not an option. And even if I could use a water bandit and do it anyway, they often have limited water at those campgrounds. If I can bring enough water for my needs, then that means a tent camper can use the campground water for their needs.
And I use my tank even at places with hookups. I LIKE my tap water at home. I don't drink bottled water (some brands actually bottle the same water that comes out of my tap). I drink it, clean with it, cook with it.