I always turn the pump off after each use. If we are hooked up at a campground and we leave for awhile, we shut the water off. If you are using your 12 volt water pump and you ever notice that it just barely cycles on now and then and you have not turned anything on, you may have a leak. DO NOT ignore this as its an early indicator of trouble. If your water switch has been off for hours or even days and you flip it on, it should not cycle on as there should be pressure in the system. If it comes on as soon as you turn the switch on I would check for leaks.
There is some variation to this as the pump itself could not be holding pressure or a one way valve could leak or toilet valve could seep water into the toilet bowl. This applies to 20 year old RV's and new RV's.