Maint books can be found here:
Airstream, Inc :: Maintenance Books
If you don't see a 2005, anywhere from say 2000 through 2009 will work with a 25 as they are for the most part similar if you find the same floor plan in any give model year between the years stated...or you can contact Airstream and they would most likely send you one for your model year (possibly at a cost).
Also, when filling the water heater, simply turn off the by-pass located in the curbside storage compartment. In the compartment, you'll see the left side of the water heater. The center valve should be closed (I believe it's a right turn), the top and bottom valves open (I believe left turns). When you connect water via the city water hookup, it will fill the tank as the heater tank is in line at this point and not in by-pass. If you don't have city water hooked up, and are filling from the fresh water tank, simply turning on the pump will take water from the fresh tank and fill the water heater tank.
I am not 100% on the valve turns, so you may have to play with it a bit....that's how I learnt.
On the empty side, open the water heater door from the outside. There you will see a nylon plug and a pressure relief valve. Remove the nylon plug, open the pressure relief valve, after you step aside from the water heater as the water (6 gallons) will fly out of where the nylon plug was fairly quickly.