We cannot say beforehand the level fuel of a particular Spaceships comes with, but we like to keep things as easy as possible. You simply return your Spaceship with the same amount of fuel as when you got it. Most Spaceships run on petrol. The exception is the Cosmo VW Campervan, which runs on diesel - and it is important to keep its tank well above a quarter (preferably above half), because the Cosmo's diesel hob and diesel heater stop working once the fuel level drops below 25% of the tank.
There's no full-to-full rule at Spaceships, it's same level of fuel on drop-off as there was on pick-up. Simply return the campervan with the same level of fuel (petrol or diesel, depending on your model) as when you picked up the campervan. So this means that:
If your Spaceship came with the petrol indicator at full (1/1), you'll return it with a full tank of petrol
If your Spaceship came with the petrol indicator at 3/4, you'll return it with a 75% filled tank
If your Spaceship came with the petrol indicator at half (1/2), you'll return it with half a tank
If your Spaceship came with the petrol indicator at 1/4, you'll return it with a tank that is 25% filled
If you return the vehicle with less fuel than on your rental agreement we reserve the right to charge a fuel fee of $100 to replace the fuel. A $200 fee will apply if you return the campervan empty or very low on fuel.