Ignore the directions it gives you they are wrong, just go to the CEPSA Service Station. it's to the right of the petrol station as you drive in. You will need a hose and adapters if you want to fill up with fresh water..

Neat service place. Be sure to bring your own snake. Note: if you drive north and you go to this service station, you then have to go south again for 22 km before you can turn!

There was no hose to the tap, so had to fill the tank with bottled water. Furthermore, a clean CP at gas station.