How much water have I used?

I have an old aussie swag hard floor camper trailer and I've put a 50 litre water bladder in the foward storage well, plumbed externally to a handpump style tap on a post. It works really well for us except it's difficult to tell how much water we've used. (The bladder is under the floor, under the bed.)
Do you have any creative ideas I could use?
I thought of something like a council water meter with a ~50 litre scale inline with the tap, but can't find anything. Anyone else?

[murray]

put a flat board on top with a marked gauge
on each side this will relate to how much is in the bladder

[ed.]

Are you looking for something like this?
http://www.allergybegone.com/flowpro...low-meter.html

Originally Posted by ed.

Are you looking for something like this?
http://www.allergybegone.com/flowpro...low-meter.html

Yep!
That's by far the closest I've seen to what I'm after. Ta.

[stockobob]

R.V. Electronics have a water level indicator which camper trailer manufacturers use on their watertanks. http://www.rvelectronics.com.au/page...=3&parent2id=9

Rob