There are 4 basic methods used to make the water flow around a camper, caravan or boat etc.

1 (not very common now) the hand or foot pump
2 the switch on the panel above the sink to activate the pump
3 the microswitch on the tap that activates the pump
4 a pressurised system. A switch in the water system automatically switches on the pump and runs it until about 15 psi (1 bar) is generated and then switches it off. Turning on a tap allows water to flow out under pressure and the pressure drop causes the pump to switch on again. The process is smoothed out using pressure accumulators or dampers. These also stop the pump from rapid switching. They can be retro-fitted and multiple units can be used to improve performance.

Because there is a potential for flood with a pressurised system an isolator switch should be fitted. It is good practice to switch the pump off when leaving unoccupied or going to bed.