You'd be hard pressed to notice any difference in mileage. We regularly have to climb miles and miles of 6% grades when we go camping, and we usually carry 115 gallons of fresh water. We've carried as little as 20 gallons, but even so, our computerized fuel monitor indicates no difference in mileage at all. As a frame of reference, when we're towing my wife's 5000 lb SUV, there's about a .3 MPG loss in fuel economy compared to when we're not towing it - that's a 5000 lb vehicle with rolling tires and extra aerodynamic drag. A few hundred lbs of water inside the RV is going to be unnoticable. Our main water tank is located just behind the front axle, and in my opinion, the biggest drawback of carrying lots of water weight is the extra weight it imposes on the axle. Mileage isn't a factor at all.
(By the way, welcome to the RVForum!)
Kev