Here is a quote from the Atwood website.
"Atwood water heater tanks are constructed of a high strength aluminum.
The interior of the tank consists of a .0015 thickness of type 7072
aluminum (pure aluminum and zinc) that is fused to the core during the
rolling process. This material protects the tanks from the effects 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. Aftermarket Anode Rods are not
required and should not be used and will void warranty."
No anode rod needed.