On our 2004 26Q, there is plastic spout with a valve just behind the driver's door to empty the fresh water tank. You fill the tank with a hose and a bendy plastic hose end in the port shown by Ichabod above. The "low point drain valves " are nearly impossible to access through a small panel near the floor on the bathroom wall. I had to remove the rear dinette seat and plywood base to access, open and close the #$%^& hot and cold water valves when sanitizing the water system. (It's probably not really necessary) There is a bypass valve under the sink to keep winterizing stuff from entering the water heater. We don't need to winterize here in Socal. More modern rigs have more user friendly drain valves and accomodations for winterizing.
When you hook up your white drinking water hose to the fitting on the side of the rig, a one-way valve feeds city water into the rig but doesn't fill the fresh water tank. You should buy a water pressure regulator to protect your plumbing from over pressure. Goes between hose end and city water fitting on the rig. You can also connect a water filter to the fitting to prevent unpleasant taste from camp water.
