hippi
rusty nuts
, spray loads of wd about.
You will need to access top of tank (through inspection plate in the floor) so you can get to feed and return pipes, sender connection....., these will be covered in muck and corroded....., take care not to snap them off or damage threads, etc......
The fill pipe is best accessed via the side bulkhead lining (in the back, from inside) from the fill cap end....
Chris