While I do not have an 05 Discovery, I can tell you from experience that the bypass valve is usually inside the coach in close proximity to the water heater.
Some examples I have seen: Under the sink in the master bathroom. behind a drawer under the washing machine. Behind a wardrobe drawer, Behind a removable panel (does not take tools to get to it,) the panel is velcro'd to the inside of a cabinet or closet.
And in a rare occasion I actually saw the hot-water bypass valve clearly marked in the wet bay of a coach.
If you find one that looks like it might be what you are hunting for, but you are not sure. Relieve the water pressure on the system, pull the water heater drain plug, then flip the found lever. If when you turn the water back on and nothing comes out of the open water heater drain, you've found it.
While I couldn't be more specific, I hope this gives you a track to run on with your coach. If someone has personal experience with your model year, I am confident they will respond below.
