RV dump stations, campgrounds, RV parks, truck stops, or any spigot we find whose owner says "ok."
We boondock nearly 100% of the time so we carry a few 6 gallon jugs in our truck. They are easy to fill while we're out and about sightseeing or running errands, and we then pour them into the fifth wheel tanks when we get home.
We use the book "Next Exit" and the website
http://sanidumps.com to find where the RV dump stations are if we want to fill the tanks directly.
We always travel with a full tank so that when we get where we are going we can stay a week before we have to worry about finding water.
We carry drinking water separately in 1-gallon jugs. We keep them under the sink and store the empties in the back seat of the truck and refill them whenever we find a drinking water refill spot (or tasty tap water).
