On our Class A, the back side of the water heater (inside a closet at floor level) has a tube going between the hot and cold pipes to/from the heater. When open to the heater cold line, water from either the on-board pump or the city line goes into the WH. With the valve closed to the cold line, water (or compressed air or antifreeze) goes straight across, without going to the heater.
If you're winterizing by blowing the lines, you can figure out which way to turn the valve by firstly draining the heater (open the pressure relief valve to let air in as the water drains). Then hook up the compressed air source and turn the by-pass valve until you don't hear any air coming out the PRV. That's the "by-pass" position of the valve.
Some systems have shut-off valves on both hot and cold pipes.
Frank and Eileen
No longer RVers or FR owners