What size tank in our camper?

We recently bought a '76mdl camper and the fuel gauge doesn't seem to be working correctly.
What size tank, in litres does it have? If anyone can help it would be appreciated.
P.S, Why I ask is when we fill it, it goes down on the gauge as normal but once it passes 1/2 a tank it more or less falls to empty and I don't know whether it still has fuel or not.
Oh and for those with a mind like my wife, I'll pass on "just keep on driving 'till it runs out!".

Fuel tank is usually 51 litres with approx 10-11 litres left whenyou get to ornage mark.
Had 75 with exactly same problem. It is most likely at tank sender which is engine out.
Though saw 1973 1700 today which had same problem and very neat hole was cut in floor behind rear seat and repair effected. Plate with self tapping screws neatly put back on.

Warren