How do i replace grey water tank drain taps?

I have a 1996 ford legend auto sleeper and the grey water tank tap needs replacing but when I try to turn the whole tap round to unscrew it out of the tank nothing happens it just keeps turning.
The tap itself looks to be like a beer barrel type its white nylon/plastic and when I turn the tap top it just comes out of the body of the tap.
Can anyone suggest best way to replace it and what with. thanks