That valve is a 3-way valve on the suction of your water pump (blue line). The valve can chose to take a suction on either of the other two lines. One of these two lines should go to the bottom of your fresh water tank. The other one PROBABLY goes to an outside connection. If so, that's the connection you use for anti-freeze.
See the plumbing pic I've attached. Most RV's look something like it.
1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10