The couple of times we traveled with our tank close to empty, we had a heck of a time getting water. Should have known better in the southwest. We stopped at a gas station once and paid by quarters about $5 and once at an RV park that let us fill up for $3. But it was driving around finding these places that was the issue. In the RV park case we had a round trip of 40 miles from the park to get the water. I've since learned to carry "devices" to get water at various pressure less spigots, but I still prefer having the insurance of our own water.
Our fresh water is 30 gallons, our black water is 30 gallons and our grey is 36. So we carry an extra plastic container of 5 gallons outside on the storage rack and a 3 gallon drinking container under the sink.