OK, I've been meaning to do this for a long time. This question has been skirted, partially explained and answered incorrectly for a long time....and AS has NEVER put a good explanation in any manual that I know of.
Everything in the explanation will revolve around these two pictures. This is a 2007 classic, but I believe any AS with the outside water compartment down below the floor, in the curved under belly would be the same in operation, although some of the components have changed.
PIC #1 (outside water compartment)
PIC #2 (water heater compartment)
Components and description: Outside water compartment
1) city water hose hookup (far Left, white in color)
2) low point drain, water heater drain, outside hand held shower hookup (brass household hose bib)
3) Black tank flush (far right around the corner, black and chrome
All of these components are below floor level and outside the frame rail. As such there is ZERO freeze protection for all of these components. When hooked to city water, the outside hose bib/low point drain is tee'd off the fresh water hookup directly behind the mounting panel. The other leg of the tee, goes through the floor, up into the undersink area by the water heater. From there that line feeds the entire trailer fixtures, including the water heater.
Components and description: Water heater compartment
1) Far left edge of pic = water line from outside water compartment which feeds entire trailer when on city water hook-up.
2) Chrome valve, front center of pic = outside water compartment shutoff, for use in freezing temps AND while using tank water/pump.
3) bypass line and one way valve (brass) = runs behind and around the chrome shutoff valve. This brass one way valve allows water to flow from left to right in the bypass lines, but not right to left.
Summer operation while on either city water or tank water: Keep chrome bypass valve open.
1) city water will feed the outside spigot/drain and the whole trailer.
2) while on tank water, outside spigot will still be fed since chrome valve is open.
Winter operation while on tank water: Close chrome bypass valve; open the spigot/drain, and use low pressure air in the city water hose connection and let it blow out the spigot/drain. Leave the drain/spigot open during freezing weather.
Since the chrome valve is closed and the bypass valve will not allow right to left flow of tank/pump water, the entire outside water compartment is now empty and isolated from pump pressure and you will have no damage to the outside components.
1)Spigot/drain can be used for an outside water source whether on city water or tank water in above freezing temps.
2) the outside spigot makes an excellent water heater drain, eliminating the need to screw around with the plastic drain on the water heater tank.
3)spigot/drain is an additional low point drain for winterizing (into storage).
I think that about does it....hope this all makes sense.