I suspect you have a Pressure valve in the water inlet cold side this is most likely the cause.
It's white has knurled knob on top and will be in the blue pipe cold feed. where the feed in to caravan is, usually under a seat so it'll be inside not out side.
There ya are and no jokes in sight.
Here's the obvious first Water tank pump connected 12 volt on and away we go.
Ensure taps are OFF.
Turn the pressure valves knurled knob clockwise to close the valve.
Turn on the cold tap half on in kitchen sink. Turn the pressure valve knurled knob ant-clockwise slowly to open the valve when the wate runs steady and you hear a clicking from the pressure valve stop turning the knob. Turn off the tap the light should extinguish after running on a few more seconds. Wait 10 secs turn on the tap check the flow is steady turn tap off light should go out.
This has to be done each time that happens I have to with my Lunar for several time I had my head under the seat adjusting the pressure valve when on hols in France this year.
Bob
September 2010