We have an 10 gal Atwood. We also have a little inspection camera that plugs into our 'phones and other devices. Interesting to look inside things.
About 2x per year, we replace the heavily corroded magnesium anode rod (Camco) in our Atwood. If you are interested in the chemistry, here is a good read. https://pdhonline.com/courses/s118/s118content.pdf
Yes, Atwoods are aluminum lined. As others have said, the aluminum is intended to be the sacrificial anode. Its long term efficacy is dependant upon the specific alloy of the aluminum and the specific alloy of of the underlying steel. In any case, sooner or later, the aluminum will have done its self sacrificing job and then it will hot water against steel. And then corrosion will accelerate. Magnesium is further from steel on the galvanic series chart than is aluminum. It will corrode, the aluminum will not, the Atwood will last longer.
Do not cheap out and try zinc anodes. The aluminum will corrode first. (see the galvanic series chart) Zincs work great on steel, bronze and CuNi -- as in sea keeping vessels.
Fresh magnesium rods help us sleep better. Others may have other sleep aids.
Fair Winds and Following Seas