I am not sure what it is like for a more southern crossing, but on the Trans America I think the longest stretch between water was about 80 miles. I would think that on most US routes water is available at least daily.

Places where it is a long way between water stops I plan where I stay based on those stops. If at all possible, I plan a longer day to get from water stop to water stop rather than overnight without having water stocked up. Some times that means a short day before or after to make the distances work out. My point is that on most trips in the US you only need enough water to get you to your destination for the day with a little extra for safety.