I've managed to clean a couple of tank out in the past using the following procedure:
Emty tank.
Scrub tank using a bottle cleaning brush tied to a bamboo cane.
Hose the inside of the tank down.
Suck the dirty water out of the tank using a carpet cleaner or wet/dry hoover.
The bottle cleaning brushes handle is made of wire, so it can be bent to reach places that may appear inaccessible.
I hope that this suggestion proves helpful, Andy.