Go talk to someone who sells hardware used by folks that pump water from a well. Also recreation vehicles ( caravans in you area?), and boats that have a 12v water supply.
They both use small pressure accumulators to hold pressure so the pump does not need to turn on and off so much. They are a small rigid container about the size of a liter thermos bottle. They have a rubber bladder in them that holds air under pressure. When the well /RV pump comes on it pressurizes the water and squeezes down the bladder. If you use just a small amount of water it comes out the faucet due to this pressure provided by the bladder and the pump does not need to turn on.
Let me look around and see if I can find a link to one.
Here's one so you know what your looking for.
http://www.depcopump.com/catalog107/16.pdf
http://www.johnson-pump.com/JPMarine...ccessories.htm
http://www.pplmotorhomes.com/parts/r...-pump-tank.htm
Larry