What does this valve do? 2019 GT 27 FB
Hi there, you are correct that the yellow valve is your water heater bypass.
Your lower positioned white valve is a low point drain. You will treat this the same as the two brass low point drains that are under the trailer near your fresh water tank drain. It is now in the open position. Turned 90 degrees will close the valve ie normal operation for water use. You will open this valve to purge residual water prior to adding rv waterline antifreeze or open it during an air purge if that is your preferred winterizing method.