Basically you have to take the tub out of the boot (easy) just make sure you get some silicone to seal it back in again or it will leak once you break the seal.
I am not sure you can fill it without taking the tub out, but I am sure you can but it will require a lot of jacking and crawling round on the floor, maybe take it to a garage with a post lift might be the mid ground easiest way of doing it.
See here for Loz's approach without removing the tub, might get a fair bit over the floor: