I believe the tank capacity is very close to 5 us gallons. It will require you lifting the cargo cover, removing the spare, removing the rubber plug that seals the floor and then unscrewing the def cap. You will need a funnel or else make a mess with the jug nozzle. Wipe up any spillage asap as it crystallizes quickly.

 

If you use a proper bottle or filling method, you won't spill any.