There was a good thread on this a couple of years ago: Click here.
Partway through the discussion, one of our members (MikeM) showed us the text of a letter he got from Atwood on this topic:
"Anode rods are unnecessary in Atwood water heaters. Even though manufacturers of those devices claim they are made for our water heaters, we do not advise using them.
"The Atwood water heater tank is constructed of a core of high strength aluminum. The interior of the tank consists of a 15% thickness of type 7072 aluminum (pure aluminum and zinc) that is fused to the core during the rolling process. This material protects the tank from the affects of heavy metals and salts found in waters throughout the country. It is anodic to these heavy metals and acts much like an anode in a steel glass lined tank except it will last much longer."
Also, if an anode rod is used, many times the metal that used for the threads is incompatible with the aluminum and differential metal corrosion occurs. The anode rod then becomes frozen in place and is difficult, if not impossible to remove.
Anode rods are only necessary in glass lined tanks."
So not only is there no need for an anode rod, but installing one could possibly cause serious problems.
The thread also included a lot of good ideas about replacing the plastic drain plug of the hot water heater with a simple valve system to make winterizing easier.
Good reading all around. There are a couple of other threads that popped up when I searched on "anode" (without the quotes), but the one mentioned above has all the info you'll need.
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