This method that I use now is a bit difficult to explain but those of you who can visualize will catch on quickly. I must admit this is not a new method and I picked it up on some RV forum.
Using a water container(7 gal container) and the winterizing, place the hose you would normally use to supply RV antifreeze to your pump in the water container. Turn the valve on. Go outside to your outdoor shower and remove the shower hose. Replace shower hose with a length of hose that will reach your input area where you fill your tanks. Turn the outside shower cold water valve on. Go back inside your RV and turn on the pump. This method will pump water out of your water container through your system into your onboard water tank.