I tend to agree with you with reference to the "sacrificial anode" vs the "sacrificial tank lining" I too would much prefer a replaceable rod rather than replacing the tank.
Review this:
http://beamalarm.com/Documents/atwoo...eshooting.html
Here is a relatively good "troubleshooting guide" for Atwood heaters. It covers pretty much anything that can (and does) go wrong with Atwood's HWH. About 2/3 down, there are comments on tank corrosion and sulfucation in the tank. He indicates that the tank is actually a "two layer" aluminum with the outer skin being made of strong 7072 grade aluminum and the inner layer being made of a softer aluminum and zinc mixture. That, I believe, makes the entire tank lining "one big anode."
My agree with you, using an anode rod can not hurt, and may even slow the process of tank corrosion/deterioration. It would seem that if the anode rod is "less noble" than the tank lining, the anode rod would corrode first, but on the other hand, if the anode rod is "more noble" than the tank lining, it would corrode slower and much less than the lining and give a "false sense of security.
In other words, zinc is the reason the anode rod corrodes first, it is "less noble" than the aluminum. so, if the tank lining is made up of 15% zinc and the anode rod is made up of 80% zinc. the anode would corrode first. But, if the anode rod is made up of 10% zinc, it would still corrode, but the tank lining would corrode faster because it has more zinc, thus giving a "false sense of security."
I'd be curious to know what Atwood's real take is on anode rods. As you say, they really don't address them in the owner's manual.