Learn to use your fresh water tank and manage your water usage.
When running on our fresh water we run out of that before either our black or gray thanks come close to full 70 fresh 70g/50b with care that can last a week for the two of us. Hand sanitizer cuts down on hand washing, saving dishwater to flush toilet.
We have done that a couple times though we seldom spend more than three days without W/S hookups. Three days camping we never even worry about filling tanks or running out of water if we dump and fillup going into a park.