H2O heater Anode...Zinc, Aluminum, Magnesium ???

Zinc reacts the quickest providing the most protection BUT haven't seen any 'zinc' rods for rv water heaters.

Magnesium reacts quicker than aluminum BUT can react to bacteria in water and cause a sulfur smell

Aluminum is slowest reaction which means rod lasts longer and might not provide as much protection. Also deposits can build up on aluminum rod making it harder to remove thru drain hole.

Most rods available are aluminum & magnesium alloy.

Anode rods are available for Atwood Brand water heaters BUT they are NOT needed, MFG. doesn't recommend them.

Suburban Brand water heaters use an anode rod (9" X 3/4" some are 9 1/2" X 3/4") and are aluminum or magnesium.

I think Suburban uses magnesium. Atwood uses none.

No need for one on an Atwood.
When I need one for my Suburban I just get what ever they have at the RV store.

Let's start with some facts-- what brand water heater do you have?

Atwood and Suburban have COMPLETELY different needs, as they use completely different materials for the tanks.