30Mallard,
Maybe I'm reading you OP wrong. You indicated that the trailer had been on city water for eight years and you found your fresh water tank empty.
You know there are two separate water systems on your trailer? With city water and city water pressure, your trailer is fed water to all faucets and toilet.
If you use your fresh water tank (with not being on city water), you need to turn-on the water pump to pump water from your fresh water tank to all faucets and toilet. Without using the pump, there is no pressure to push the water through the lines from your fresh water tank.