i would run with some fresh water in the tank as well. in the past (pre 2004). i ran dry. we pulled into a campground in nebraska, went to eat, when we got back, they had lost the well. first and only time i stayed in a full hook up campsite that didn't have water. first time i have stayed in any campground without water.
i've never noticed any change in my fuel usage from carrying fresh water or some water in the waste tanks. when i added a heat pump to the front, didn't see anything there from the additional drag. i weigh 11000 (maybe a little more) dry. that may be one of those things that u know should affect milage and can calculate potential extra fuel usage, but can't actually see it in practice. apparently if is is a very small part of the total weight or wind drag, it isn't noticable.