If you crawl underneath, you should find the fuel tank held up by a couple of hangers or straps that hold it to the frame. I'd try to drain the tank first, it'll be much lighter. Support with a jack, unbolt the hangers and lower the tank.
You didn't say whether it's gas or diesel. On a gas tank, it's safer to drain gas and fill with water to reduce explosion. On a diesel, the fuel is much less volatile.
I've used rope nylon straps to hold the tank up until I could reach up and remove the connections. In the case of just putting on a new fuel line, that might be enough. If you have an in tank fuel pump, you might have to pull it all the way down.
Here's a video on removing a gas tank from a pick up, it shows many steps in the process.