For weekend outings I've never come close to running out of water. For longer outings, I put fresh water in whenever I need to empty the gray water. I have a collapsible hand cart I use to get the 5 gallon gray water container to a place to dump it I carry the empty 7 gallon fresh water container in the hand not pulling the cart. After dumping the gray I fill the fresh, put the fresh on the cart, and carry the empty gray.
I figure the extra two gallons in the fresh water container (7) over the gray container (5) will more than make up for whatever water we use that doesn't get put down the drain.
So far the logic is working. We camped all last week, and I never had to give a thought to the fresh water level. I always had some left in the 7 gallon container by the time the holding tank was full. As I neared the end of the week I just emptied the gray and didn't fill the fresh, so that I wouldn't be dumping as much out of the holding tank and onto the ground on the way out of the campground.