When going to a location close by that I know will not have a water source I fill the tanks at home. When going to a location more than 50 miles or so away I will fill it up someplace closer to where I am going. Sometimes you need to ask the locals - some full service gas stations will have a water tap - some rest areas as well. If camping at an actual park that has designated camping sites but no water at each site they will often have a water source someplace in the park or a dish washing station.
I carry an extra hose so I have lots of length and a Water Thief
- it has come in handy a number of times when the water source has no way to screw the hose onto the tap.