If you're going for smoothness and want to use tween, you'll need to define an "update period", as tween is something that happens over time.
You wouldn't tween something every tick, since that is no different then setting the position directly.
You also generally don't want to use multiple overlapping tweens unless there's some specific effect you are going for.
So to prevent overlapping tweens, such as your fuel draining normally, and your fuel filling from pickup, you'll need some additional information. All tweens need a start point and an end point. Assuming your fuel is still draining during the time it takes to fill up, you'll want to find out how much fuel would have been drained during the time it takes for your fill up tween and subtract that from the end amount. When it's done filling, then start a new emptying tween from that point to 0. But then you would need to set a new time, based on the amount of fuel you currently have, since fuel drain should be constant...
Anyways I still think tween isn't the way to go here.
Just have a fuel variable that is subtracted from (dt amount) every tick for a constant smooth rate. And if you want your fill up to be smooth do the same thing, and add fuel at a constant rate every tick until the total amount added runs out (you can keep this in a separate variable).