Virtually no difference at all, UNLESS, you are close to you're to your max weight. It's the front end wind loading on your rig that has the most effect on mileage... think about your engine having to push/pull a brick through the air.
Once the weight of your rig is moving, the engine isn't going to notice the extra few hundred pounds of water. Speed has the most detrimental impact on mileage.
I look at a full tank of fresh water the same way I look at seat belts, you might never need it, but if you do, it's great to have it.
Several times we've arrived at campgrounds where they've had a power failure, and the water system boost pumps were out. It was great to have our own water then.