At the national park campground near the main visitor center there is an area just before you go in and it can be accessed easily. you dont have to be using the campground.

Campendium maps should show you locations. I bring water onboard, but dump at the Speedway in Payson.

You can use the dump station at the campgrounds in the village. Not far at all.

The Grand Canyon Village Market Has a frost Free valve just to the left of the entrance that will fill a 5 gallon jug in about 30 seconds. And it is tasty springwater.