Are you sure it doesn't say ďwater heater bypassĒ? When winterizing the water heater, it would be in the bypass position so you don't have to fill the 6 or 10 gallon tank with antifreeze.
The bypass valve could be anywhere in the system but I'd start looking at the backside of the water heater to start.
I'm not sure changing the water heater bypass will cure your problem of not filling the freshwater tank, but it's a start.
Denny, Connie & Shadow (former barn cat made good)
2006 Tiffin Phaeton, Cat C7, Freightliner Chassis