While this may sound backwards, I have found it easier to locate a dump place than potable water. Lots of places have it and the employees don't even know about it.
The truck stops that have an RV lane often have potable water. Many times the water tap that you can find is marked as non-potable and this can be for good reason.
Most fast food places have an outside tap that requires a key to open the valve. Often, nobody knows where the key is or has seen it in years.
Just keep looking and asking.
Matt