Fill in fresh water tank

We have a lever that can be turned to city or gravity. I turned to gravity but it didn’t fill the tank. I can only assume that gravity is only for use when tank is full but can’t seem to figure out how to get water in the tank itself. I’ve done it before but it was a long time ago and I believe I did it after reading something on here but I can’t find that post. So frustrating! We don’t normally travel with water but when we are traveling long distance, it is nice to have a little so we can stop and use our own facilities.