When I know I won't be using the Airstream for more than a month, I drain both the fresh-water holding tank and the H2O heater. Here's why.
Water from a municipal source may be chlorinated when it goes in, but chlorine dissipates over time and depending on your water source, some troublesome things can start to happen.
In the H2O heater especially, there is a tendency for some organism to take over producing a strong hydrogen sulfide (rotten egg) smell. Once it takes over, it is very difficult to kill it and get the hot water smelling fresh again. It's much easier to drain the tank.
In the fresh-water holding tank, I am concerned about the odorless, tasteless bacteria that might develop.
When I drain the fresh-water tank, I add 2-3 ounces of sodium hypochlorite (household bleach, unscented) and a couple gallons of water to cover the bottom of the tank.
If you are someone who can't stand chlorine, and only fills your tanks with non-treated water, it's probably even more important to drain.
As a footnote, we just returned to our winter home in Mesa, AZ. I forgot to drain the H2O heater when I summerized last April. The hot water reeked of hydrogen sulfide. Very unpleasant, and it took hours to drain and rinse.
Hope this helps.
2007 Classic Limited 30 (Sold)
2007Chevy Silverado 2500 HD Duramax/Allison
Four Corners Unit WBCCI #8654, Affiliate NM Unit