Water tank gauges are notoriously bad. 

I couldnt get my grey water tank to read anything less than 30% empty. 

Paid $30 for an 800ml bottle of Thetford cleaner, half filled with water, drove for an hour and left in for 20 or so hours then emptied. 

No difference, still reading 30%. 

Did a bit of searching around and found 

Dawn Platinum Advanced Power Dishwashing Liquid was highly recommended for this purpose.

Got 2 x 2.6 litre bottles (thats how its sold)

Emptied the grey tank and filled up the fresh tank.

Turned on the HWS and waited a while.

When hot, poured 600ml down the kitchen sink then pumped the hot water in till emptied.

Left it overnight and then emptied the gray water.

Big fat zero percent reading.

As its very potent at breaking down grease, a lot recommend using it as your standard dishwashing liquid all the time to keep the tank clean.

Just thought it may help others.

Woolies, Coles, eBay sell the 2x2.6 litre sets through their Marketplace for around $70-80  
Costco sell the same for under $50