Many times I prefer to fill a good source water and transport rather than taking a risk later. Water quality and quantity is suspect when you head out toward the desert verses going out to water rich areas with rivers and lakes, so there is not just one answer. I find many dump stations also have a place to take on potable water - You may need to pay a dump fee even though you are just filling up the fresh tank. Some fueling stations have RV dump and fill and include the service with a fill up of fuel.