I installed a "winterizer kit" similar to the one 1775 linked.
The idea is to turn the valve so it draws from the plastic tube instead of the line to the freshwater tank. Just sticking the tube into a jug of pink RV antifreeze doesn't work for me; my pump requires some priming. I keep a dish detergent bottle with squirter cap handy and use it to fill the tube with antifreeze, then stick the end of it into the jug which must be higher than the pump.
The same method works to prime the pump when starting to draw from the freshwater tank. My pump will prime itself from the tank when it is completely filled, especially if the RV is sloped a bit with the tank on the high end. Otherwise it needs a bit of water from that plastic tube and a bottle of water, turning the valve to the tube draw until the pump fills itself.
I am very glad I installed that valve and tube.