The mileage impact of a full vs. partially full tank will be imperceptible, but there may be a weight issue, depending on how a partular RV is loaded, and its weight limitations.
Our first Class A had a typical water tank setup - a vented tank with an overfllow. The overfllow, however, couldn't keep up with the water hose (gravity fill) and I almost blew my water tank up one day when I was filling it. Ever since then, I visually watched the water tank when I was filling it. (It was under the bed)
In all three of our Class As, when the tank level indicators were indicating full, the tanks weren't full. I could get several more gallons in them. That's no big deal if we were going to be hooked up to water somewhere, but we mostly boondock, and water is a valuable comodity when boondocking. Some RV's water tanks do siphon while driving if they've been topped off, so you'll need to determine if yours does that or not.
Kev