If you've got the winterizing connection that allows you to pump antifreeze with the pump, then there are probably 2 valves on the suction side of the pump, one to the AF connection and one to the FW tank. The one to the FW tank is normally open and the one to the AF connection is normally closed. When you want to add the antifreeze, you swap them. In this case, close the FW tank valve also, and you should be OK, at least until you want to use the FW tank.
(I've also used these to fill my FW tank. Attach your filling hose to the AF connection and open both valves and the water goes from the AF connection into your FW tank. But be careful - do it slow or you can pressurize your FW tank, which is NOT a good thing.)
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10