You have a water leak. Could be anywhere in the camper OR it could be the 1-way check valve inside of the water pump itself. The latter will allow water from the water lines to trickle back to the freshwater tank. When the pump senses the drop in water pressure in the water lines, it’ll cycle to rebuild that pressure.
You can try adding an external 1-way check valve on the output side of the water pump.
If it fixes it, that was the issue.
If it doesn’t, then you need to go find your leak.