New Casitas come with an accumulator I understand. My 2003 was before that time. The accumlator uses the water pump to build up a reserve of pressure and then when you turn on the faucet, the water pump turns on and off less frequently. Saves battery
life (I think Pete had a nice chart that illustrated that) and wear and tear on the water pump. Although water pumps appear to be pretty robust devices as it is.
As a maintenance tip in the spring, part of the recommissioning of the trailer in the spring, you should check the pressure in the accumulator with the water pump off.
I chose the area in the dead space in the corner, but location is pretty flexible and I've seen installations in the front closet as well.
I bought mine at West Marine. Water Pressure Accumulator