Ive always filled up at home, as I know the water quality is acceptible, but then again I dont use it for drinking. I can definately feel the difference in weight though, as when I bought our jayswan, I towed it home empty and hardly noticed it was there..when we had our first trip away with a full load plus full tank of water I had to wind the brake controller up considerably !
If you arent drinking the water then probably not a bad idea to fill at a location near to where you are camping.